How does a spacecraft traveling 186,000 miles/sec slow down? Would that require a massive fuel source?
Nah - the aliens simply turn around, and aim their laser at a black hole in the opposite direction. Of course, you have to figure the round-trip time of the laser directed at your initial black hole. If the black hole starts out close to you - say, about 2 light-years distant - your laser-generated thruster only starts pushing you when the laser light returns a mere four years after you turned it on.. . . and say your destination isn’t far away from your starting point - maybe five light years away. You need to operate at max thrust accelerating forward for perhaps five years, you need your forward thrust to stop - which happens at a time years after you turned your laser off - and then you need to to have started your retrograde laser long enough ago that your retrograde thrust goes to maximum when your forward thrust stops.
Depending on how far away the second black hole is, you might have to fire lasers at max power in both forward and retrograde directions simultaneously.
Piece of cake.