score:4
Funny you should ask; I've checked with my 'source' who lives in LA, works for NBCUniversal, and has a season's pass to Universal Studios.
In her view, rain means fewer Californians will go, and 90% of the time the "rain" is light and intermittent.
This week, there probably won't be a huge difference between Black Friday and Saturday, although Saturday might be preferable because it's supposed to be cooler.
Above all: have a great time.