It probably isn't a violation of the ToU... what part of the ToU would it violate?

The reasoning part. I might think of reasons that admins might put are:

"Just for fun"

"You're an idiot"

Those don't break the Terms of Use.

Well if a reasoning for a block does break the terms of use, what will happen exactly?

There is no public list of guaranteed consequences for violating the ToU. Chances are, at best, there is a pre-defined list of options from which staff may choose at their discretion. The only people who can possibly give a decent answer to your question is staff and even then, they may decline to tell you.

If you have an issue with a block, you can try to discuss it with staff. However, keep in mind that Wikia gives admins practically free reign over who gets blocked. As such, I have difficulty imagining cases in which the block itself would be a violation of the ToU. That said, I was involved in a case where the user was globally banned for ToU violations and the blocks they placed on other users during the course of a heated exchange were reverted by staff. I have a feeling that is an exceptional case though. Also note that the blocks themselves were not necessarily deemed ToU violations.

Thanks.

PS: Sorry for being a pain, but if I block someone and use cuss words to their block log, will that violate ToU?

As far as I have seen, staff now sometimes acts upon users who have been blocked on a wiki despite never editing/visiting that wiki and despite having a good user activity on other wikis, maybe as part of their new contract thing but not sure.

Almost any other block or block reason are given free reign by staff (as Andrew already said), unless the block or block reason are made based on racial, transphobic, homophobic etc nature. Just using swear words in block reasons is usually immature, but not against ToU, same as editing a wiki.

The violation needs to violate the actual ToU... not what you want it to be.