Blackhole has a time release before when you can do it again. If you can keep the blackhole active on screen (releasing it when they high in the air), the blackhole travels up higher before it starts to fall. During that time, the time release has ended and you can release a second blackhole.
When i found this, i named the move Lord of Shadow. I made an entire combo video out of it when i found it.
I was inspired by
InFlames double blackhole combo in the corner with ex df3, which then led to me finding Lord of Shadow aka "blackhole glitch"
InFlames - Double Blackhole Combo
CHECK4900 - Lord of Shadow Combo Video (Quintuple Blackhole Combos)