Modernity and black magic can coexist?

If your really afraid of black magic in Thailand remember,if you don't bear any ill will it "bounces" back to the owner.In other words if your a good person or virtuous someone's ill will can not harm you.Not even a powerful nature spirit can harm you if your heart is pure.A virtuous person is respected by other beings.Take refuge in your virtue.Your sila gives you the confidence to stand up and face hostility because your blameless.Your whole.I advice that you recite the Paritta Protection Book.And do a lot of metta practice.If someone is going through great lengths to spread and concentrate I'll will on you.You should be doing the opposite.For hatred can only be overcome by love.

TL;DR version:

It's possible to protect oneself using the foundations of mindfulness (satipatthana), virtues (sila) and by not having bad intentions.

I think the general idea is that through these, one would generate good karma that takes one away from falling to black magic.

Long version:

There's a discussion on protection through Satipatthana by Ven. Nyanaponika where he quotes SN 19:

This is the right way," said the Blessed One and spoke further as follows:

"It is just as the apprentice said: 'I shall protect myself' — in that way the foundations of mindfulness (satipatthana) should be practiced. 'I shall protect others' — in that way the foundations of mindfulness should be practiced. Protecting oneself, one protects others; protecting others, one protects oneself.

"And how does one, in protecting oneself, protect others? By the repeated and frequent practice of meditation.

"And how does one, in protecting others, protect oneself? By patience and forbearance, by a non-violent and harmless life, by loving kindness and compassion."

In the Atta-rakkhita Sutta, there is discussion on self-protection through virtues (sila):

Those who engage in bodily misconduct, verbal misconduct, & mental misconduct leave themselves unprotected. Even though a squadron of elephant troops might protect them, a squadron of cavalry troops, a squadron of chariot troops, a squadron of infantry troops might protect them, still they leave themselves unprotected. Why is that? Because that's an external protection, not an internal one. Therefore they leave themselves unprotected. But those who engage in good bodily conduct, good verbal conduct, & good mental conduct have themselves protected. Even though neither a squadron of elephant troops, a squadron of cavalry troops, a squadron of chariot troops, nor a squadron of infantry troops might protect them, still they have themselves protected. Why is that? Because that's an internal protection, not an external one. Therefore they have themselves protected."

And finally protection through not having bad intentions in Dhammapada 124:

The bhikkhus then asked the Buddha, "Venerable Sir, is the wife of the hunter who is a sotapanna, also not guilty of taking life, if she has been getting things like nets, bows and arrows for her husband when he goes out hunting?" To this question the Buddha answered, "Bhikkhus, the sotapannas do not kill, they do not wish others to get killed. The wife of the hunter was only obeying her husband in getting things for him. Just as the hand that has no wound is not affected by poison, so also, because she has no intention to do evil she is not doing any evil."

Then the Buddha spoke in verse as follows:

If there is no wound on the hand, one may handle poison; poison does not affect one who has no wound; there can be no evil for one who has no evil intention.

In Buddhism, does black magic, magic spell exist?

I believe so... the Buddha seems to recognize some pretty powerful black magic, e.g.:

“What do you think, householder? Suppose some recluse or brahmin came here possessed of supernormal power and attained to mastery of mind, and he spoke thus: ‘I will reduce this town of Nāḷandā to ashes with one mental act of hate.’ What do you think, householder, would such a recluse or brahmin be able to do that?”

“Venerable sir, such a recluse or brahmin possessed of supernormal power and attained to mastery of mind would be able to reduce ten, twenty, thirty, forty, or even fifty Nāḷandās to ashes with one mental act of hate, so what does a single trivial Nāḷandā count for?”

-- MN 56 (Bodhi, trans)

If it is, what's our belief system should be when we are taught to treat all with loving kindness?

I don't believe this changes our imperative to treat all with loving kindness...

"Monks, even if bandits were to savagely sever you, limb by limb, with a double-handled saw, even then, whoever of you harbors ill will at heart would not be upholding my Teaching. Monks, even in such a situation you should train yourselves thus: 'Neither shall our minds be affected by this, nor for this matter shall we give vent to evil words, but we shall remain full of concern and pity, with a mind of love, and we shall not give in to hatred. On the contrary, we shall live projecting thoughts of universal love to those very persons, making them as well as the whole world the object of our thoughts of universal love — thoughts that have grown great, exalted and measureless. We shall dwell radiating these thoughts which are void of hostility and ill will.' It is in this way, monks, that you should train yourselves.

-- MN 21 (Buddharakkhita, trans)

2. Supposingly all these exist, what's the odd of being the target apart from jealousy?

I don't think this question is relevant to our site.

Supposingly if you are very much enlightened and practice meditation vigorously, does this make you any less of being the target?

Maybe... to some extent enlightenment protects one from harm, but mostly it protects one from reacting to harm and therefore causing further retribution. The stories of Angulimala and Mogallana both exemplify this idea.