Just for the record, if you'd want to be mediocrely realistic, the black hole would (assuming you don't feed it enough anymore) shrink in size rapidly while emitting rapidly increasing amounts of (hawking) radiation.
Ignoring any coolness constraints, this could maybe result in:
1. Heavy pull effect (since singu is no longer shielded) that diminishes over time.
2. DPS that builds up rapidly and maybe ends in an explosion.
The gameplay impact would be ok I think, it would draw in units at first so they are exposed to the heavy subsequent damage.