I think deck strength, element strength, and current player preferences, i.e. metagame, all play a HUGE role in what makes a card OP/balanced/UP.
Player preferences seems to be the biggest thing getting cards in the nerf section in the first place. If a card is A.) used in a popular 'good' deck, or B.) shuts down a popular 'good' deck it seems it ends up here. Supernova, Graboid, and fractal fit in 'A' and Black hole, Hope, and Earthquake fit in 'B'. If supernova powered speedbow's weren't a hugely popular deck in PvP and the game had a larger mono/duo scene, no one would complain about black hole being too powerful. 3 quanta and 3 healing are an afterthough to a mono user. The current metagame is what makes black hole so annoying, not the overall power of the card. The card was designed to counter rainbows and it does a perfect job at what it was designed for. If black hole is annoying switch to a different, non rainbow deck and you will not have the same feeling.
An opposite example of this is pufferfish: its commonly agreed up that its a powerful card, but noone uses the card,so it gets buffed into a more powerful version of itself. The card probably still wont get use in the game as powerful as it is due to the fact that there are so many faster decks in the metagame and slower decks are at a disadvantage from the get go.
From a gravity perspective, it needs black hole based on the cards that go around it. Mono gravity is really a painful element to use. Gravity is a slow element without any hard hitters other than charger. It doesn't have a lot of great things going for it other than momentum, oty, and black hole and it doesn't really have that many great targets for momentum. Its only CC, gravity force is useless if momentum is out. Black hole makes mono gravity strong against 1 thing...rainbows. Pack chargers, otys and black holes and you will lose to a lot of decks, but you will stop rainbows cold. I always look at gravity as the anti-rainbow element because that's about the only thing it is really great at as a mono.
The only strong deck that I've seen that utilizes black hole is the flying discord black hole deck. This deck sounds great on paper and sometimes is. I've beaten opponents without them being able to play a single non tower card. The deck can also epic fail geatly. Surprisingly...it is weak to rainbows. Even with 6 black holes, it doesn't have enough other cards to keep up with a rainbow since the discord are not causing a stall. It is also weak to quanta denial, and CC (for 1 quanta, shockwave, fire lance and lightning bolt can all kill discord).
If anything, we need more cards like black hole to shift the balance in PvP away from rainbows and we'll see that this card isn't that OP at all.