I am seeking advice for blasphemous intrusive thoughts

In short, it's a rare predicament but i doubt that you are the first or the only person experiencing this.

People have to deal with all kinds of conditioning and the general framework is universal.

Lust can not be contained because it's unreasonable in nature. People are lusting after the inappropriate and socially unacceptable, fondness of form it is not called a perversion of view for a honorable expression.

In Buddhism all fondness of form is to be abandoned in it's entirety, the object of the desire is of no fundamental relevance.

Some people lust after wives of friends, family members or dead corpses, these predicaments are very common.

As it actually is lusting is shameful period. It is socially accepted to lust after certain people but not another.

I am not saying it's all the same but in the Dhamma it's all wrong and shameful as it's an act that cultivates passion which is a cruel thing to do towards oneself and another whoever it may be.

Lust when cultivated towards the acceptable eventually just burns like a wildfire and is not going to be contained, it doesn't conform to these arbitrary restrictions of acceptable & unacceptable as it is a corruption based on delusion and it is not a self nor personal, it simply begets irrational behavior.

Perception of unattractiveness counters lust.

From what i know, the merits you acquire by focusing on solving these issues can potentially negate going to hell as you can theoretically become even a non returner by abandoning lust completely not to speak of lower attainments and going to heavens.

That being said i guess that if you are going to spend your life being devoted to attacking with the hand then you are probably going to feel bad because of it in this life and the next.

But is a blasphemous thought towards a Buddha or bodhisattva really bad?

Yes, (but not so much need to get into in regard of "bodhisatta", a Bodhisatta isn't a good refuge at all), nothing, or less, more bad. Such may happen, and of course is heading on to hell. Yet, it fault is seen, if having made amend toward the Gems, taken Refuge anew, and resolve not to go after such in future, makes one pure again and able to walk on.

As for general avoiding, there is nothing more of help then proper assosiation and avoiding all which deals with doubts, give into specualation or even critic toward the Gems.

Pardon and Vandami (Veneration) is a practice usualky made regulary, mostly in evening chanting and before ceremonies, Sila-days. Heal refuge is very importand, otherwise one is off the track.

Good to seek out for a mature Parisa and to avoid the many destructive "Dhamma-places" within commercial medias and under doubters.

Bhante Thanissaro shortly translated a very related Sutta on that matter: The Great Lion’s Roar Discourse - Mahāsīhanāda Sutta

[Note that this isn't given for stacks, exchange, other world-binding trades, but for escape from this bond.]

I would suggest you to handle the issue from different aspects.

If you have been suffering with obsessive compulsive disorder it's not bad to see a psychiatrist. Some times a small pill can alter the brain chemicals and can give you a great relief if there is a real disorder.

Secondly keep your spiritual practice(Reading, contemplating & meditation) consistent without forcing yourself too much.

As chris pointed out and also from your own experience the way the thoughts pop up is random and we may not have much control over it. But consistent spiritual practice will help in reducing the frequency of occurrences. Slowly you will have the wisdom to see things while they are happening without being over powered by them. It's a gradual thing and isn't going to happen overnight. Even when over powered by obsessive thoughts, accept the failure without being too much self critical and continue your practice.

Bodhisattvas are not mere physical beings, they are embodiment of infinite compassion and wisdom.

Finally reflect on the story of Angulimala and be out of too much self criticism. Though he was a brutal murderer he was saved by the wisdom of buddha. We are all in the same boat.

It is plain from your post that this has troubled you deeply and that you have waged a long struggle with obsessive thoughts. Let me first express empathy to you and support for your effort to heal what troubles you.

It’s critical to acknowledge that when we are sexually aroused, all logical thinking goes out the window, and through that open window any number of attractions may enter that we later, with clearer minds, find to be deeply disturbing. It never helps us to simply recoil in horror at our own minds, we have to look at what’s happening there. There are many possibilities. Perhaps a feeling of guilt over masturbating clashed unconsciously with a desire to be pure (represented by a Bodhisattva) and that tangled illogically into a sexual fantasy. Perhaps a sexualized Bodhisattva was seen as taboo and therefore exciting, as there is a power rush that comes from breaking sexual taboos. Perhaps your mind was unconsciously testing your own limits of faith to see if you’re willing to degrade a revered personification into a mere sexual aid. Perhaps it’s a mix, perhaps it’s none of the above, but I think it’s worth reflecting upon to try to determine why this thought arose. However, again I stress, don’t expect a totally logical answer. Sexual excitement is not logical, it’s sensory and emotional, which is why we can easily look back at our sexual acts and ask, “Why on earth did I do that?” The answer is because now you’re thinking logically and earlier you were following sensations, and those are two completely different thought processes.

In the end, trying to untangle the “why” is not as important as “so what do I do now?” The core effort for Buddhist laypeople in regard to sexuality is to be constantly aware of rising sensations, remove the fuel if they’re leading in an unhealthy direction, and only allow sensations to progress if they’re leading in healthy, loving directions. Sensations arise in everyone, whether you have OCD or not. The challenge for all of us is to do exactly what you said you’re already doing: “just maintain awareness of them” and let them go. But we can’t do that when we’re caught up in sensual pleasure, literally in the act of masturbation. Had that troubling thought come to you in a calm moment, probably you’d note it and let it go. But once we’re aroused and in the act, it’s much harder to stop the train of thoughts. So what to do now? I bet you already know the answer, it just helps to hear it from others: don’t beat yourself up, don’t be angry at yourself, it wasn’t something you intended to do. Recognize that illogical and disturbing attractions can arise when we give in to sensual pleasure, so we must be skillful at the outset to direct our sexual lives in a loving, healthy direction. When we consciously set that direction toward loving relationships (or if you’re single, on loving fantasies) then there’s much less risk of being pulled off course.

Your fear of a hell rebirth over a single bad moment feels to me like a Christian-influenced punishment mindset – do one thing wrong sexually and that’s it, you’re going to hell. Karma is not punishment meted out by an angry Other. Karma is the cause and effect results of your thoughts and actions. If you obsess over your failures and feel great anxiety, you’re putting yourself in hell right now. If you consistently and intentionally belittled personifications of goodness into sexual objects, that thoughtlessness and slavery to craving would leave you no better than an animal or hungry ghost and perhaps cause such a rebirth. But that’s not you. That’s not what happened. You had a momentary failure and now you’re doing the right thing, you’re working to understand it so you don’t repeat it. People who do that consistently and genuinely don’t need to worry about a bad rebirth. In life, as in meditation, we must constantly work through these failures of focus and intrusions of desire as we progress to stages where we master them more and more. Hope this helps, and best wishes to you.

Let me ask you this - which is more powerful, the original thought or the regret and repentance you have for having thought it? The stronger one is more determinate of your karma than the weaker.

From my own experience, anything I think about -- e.g. people, places, feelings, bits of text -- is going to be recalled or come to mind sometimes. My mind produces bits of imagery more at less and random, like the waves on the ocean, little memories arise and cease.

So if you think about sex (or objects of lust), and about Bodhisattvas, I'm not very surprised that occasionally you remember or imagine both at more or less the same time.

I think that's an autonomous part of how the brain works -- it picks things (memories) more or less at random, and maybe tries/imagines them together to see if they fit somehow. And so the mind makes new connections -- apparently, I think that modern science says, that's part of learning and remembering and dreaming.

If you don't want thoughts of sex to arise at inappropriate times, you might find that would happen if you stop making a habit of thinking about it and wanting to think about it.

In summary I suppose that the fact you imagined these is the fruit of past kamma i.e. the result of what you thought about in the past. And the thought wasn't intentional, and you developed no further intention to act as a result of the thought (or the only "intention to act" that you developed was the intention to post a question about it on this site), perhaps to that extent it wasn't developing new kamma.