I think Ivory Black is, at least in my opinion, the most common, all-purpose black that will turn gray when mixed with white. If you're looking for an even more neutral gray, you might consider Mars Black. If you're looking for a really rich, deep black, you could look for Spinel Black (I think only Gamblin makes this).
I find the Spinel black to be too dark for Bob Ross paintings, but it would depend on your approach. If I were only going to have one true black paint (not the more chromatic Bob Ross Midnight Black), I would probably go with Ivory Black personally.