No, there is no Marvel blacklist.

But there are certainly creators who, for one reason or another, nobody is in a particular rush to hire.

Sounds like the same thing, but it isn’t. In a blacklist, nobody could hire a creator even if they wanted to. In the latter, nobody in a position to hire a creator chooses to do so. Different thing entirely—and one whose resolution is much more in the hands of the creator in question.