Hmm, seems... odd. Really cheap PC that lasts 4 turns at a cheap cost... even if it had no ability it would be considered a good card for Gravity, having 15HP at a cost of 3 quanta. Seems like it almost balances itself with the turning-into-BH, though that seems almost useless if the holder is using Gravity, as the card itself is better than the BH it will produce when over 50 HP.
EDIT: Imo, to make it more geared towards
(as producing a BH isn't really a thing you want to happen if playing that card), why not make it something like "And if your mark is
, it turns into a BH at (insert higher number) HP", or even if your mark is
, it gains less HP per turn or something