The only reason we can see black holes in the universe is because some of them swallow up gas and dust.  This heats up material that is spinning rapidly around the black hole as it falls in (called an accretion disk), and produces massive jets of material due to conservation of angular momentum that can be seen across the universe.  The energy released in the jets and the energy given off in the accretion disk are proportional to how much gas and dust is being consumed by the black hole.  More matter = more food = more energy released.  But...