Black Holes CAN be devastating, but I don't think it's overpowered just because it almost never provides both quanta denial AND healing, unless whoever its being used against has an extremely bizarre, low-quanta rainbow deck. Even against my rainbow rush decks, if a black hole is used later (after I've dealt 30+ damage and it's actually providing good healing), the quanta denial is pretty insignificant. It's really only good for quanta denial against a rainbow deck in the first 1 or 2 turns, and at that point, it's probably not healing very much. It's a versatile card, but it's very, very hard to use to it's full potential (without playing a very expensive gravity nymph to use it once per turn), which is why I don't think it's overpowered. Besides, in MOST games, where the opponent isn't rainbow, it's only going to take away 6-9 quanta.
It has a situation where it is overpowered, but only when the player using it has lost a bunch of life AND their opponent has a rainbow deck AND their opponent has few of each quantum and still needs them (even though they've already dealt a bunch of damage).
Almost all of the time, though, Black Holes provide just a small boost. I think it's in line with Phase Shield and Wings, where they can be devastating if you are facing the right opponent, but a lot of the time they will be ineffective. It's a trade-off.
I think Black Hole is balanced.