I really don't believe that this card is overpowered. I own a gravity nymph, and have tried a few flying discord/black hole decks and while the concept is great, it simply is too unreliable of a strategy to be effective. To work against a mono or duo you will need to draw discord and multiple black holes or a nymph (and probably a quint too) to get going, while also trying to get the quanta necessary to play all those cards and fund the nymph.
The way I see it is if you happen to have a lot of black holes or a gravity nymph in your deck, more often than not you will fight a mono, duo, or trio deck in which those cards' use will be limited. If your opponent is running a rainbow, tough luck for them, you have a massive advantage. Its a tradeoff, black hole sucks (compared to the next best card, we're talking opportunity costs here) under the majority of circumstances, but is extremely situational.