As much as I hate this card (because I like playing rainbows :p) I would have to say that it doesn't need a nerf. It's a rather situational card, as it only really shines against rainbows or decks with a lot of different types of quanta (rainbow, trios, immorush, etc) or else decks that use very little quanta or duos off the mark. (pdials, splat, chaos wyrms, etc) Rather than nerf black hole itself, I'd like to see more spells or other ways to get quanta in the style of nova or immolation, to give ways (particularly to unupped play) of getting out of a black hole lockdown, so I guess I could vote for creating counters in that sense.
It's mostly a topic for another thread (though it does have some relevance here) but I think that amber nymph does need a nerf. One black hole isn't a serious problem balancewise, or even a few, but spamming it every turn can devastate even monos. (it's like 3 pests in 1 card) I've had so many people ragequit against amber nymph, even in league play, it's just ridiculous :p No other card can lock things down quite so badly as amber nymph. There are FGs that I can literally stop dead with just this one card.
Also, sanctuary doesn't suck in rainbows; I use it there all the time, partially due to BH and silence, but also due to the healing. (but then, I love playing rainbow stalls) I've even used 1-2 sanctuaries in speedbows before, where it surprisingly doesn't suck.