This is what I get from blizzard support answer:
"The kinds of things that can get an account banned are the ones that provide some kind of in-game advantage or automates a part of the game so it can be done without a player.
If this mod isn't doing those types of things, it should be fine. I'm afraid that we don't keep a list of addons that are valid, and those that are not, so I cannot say for sure whether Sweetfx is allowed since you know what it is, and how it works, you should be able to work out whether it fits the statement above :)"
so sweetfx should be fine to use with blizzard wow