Supernova can only be used once per turn, and that's slowed down the decks considerably, yet they're still everywhere. Apparently discord/BH has been a problem for years now, and we all know its crippling synergy (hence its abuse). Look at it this way: to stand any chance at all in the arena, in PVP, or against a false god, you need at least a duo deck. So, the mono argument is completely thrown to the wind as situationally useless.
I guarantee that if BH is nerfed, it will still remain a hard counter and cripple rainbow decks. Let's look at this analytically, shall we? Given the advent of the SN nerf, one can generate 4 quanta every two turns. BH would be able to drain 3 quanta ever two turns. Seems fair, yeah? Now, let's look at quantum towers: in order to generate 36 quanta, with 3 in each element, we can assume you need 12 QT for one turn. Most rainbow decks that aren't a deckout run ~ 5-7 QT, so let's say 6 QT. In two turns, you generate 36 quanta. Every two turns, bam, BH drains 36 quanta. I don't see the problem in my nerd suggestion. As it is right now, BH walks all over the only viable decks in competitive play. Why not tweak the balancing? This is still neglecting the utter chaos that an Amber nymph can cause, but it's susceptible to CC, so we'll let it slide. And Sanctuary is hardly used enough to be considered a hard counter for these scenario, since your quanta pool is crippled before you even get a chance to play it.
And this is assuming you have all your QT out. Seriously, this is hardly a crippling nerf to BH, but more like putting it on even grounds with its generating counterpart.