When two black holes eat each other, they release a lot of energy.
A lot. A significant fraction of the mass of the black holes is converted into energy, radiating away as gravitational waves, ripples in the fabric of spacetime. These can have as much energy as — and I am not kidding you here — tens of thousands of times as much as the Sun will radiate over its entire lifetime… and they blast away in a few seconds. Put another way, during that time the black holes emit hundreds of millions of times more energy than all the stars in the galaxy combined.
That kind of power is terrifying. Mind crushing. Yet, for all that, the event itself is entirely invisible, giving off no light at all. It's possible under some specific circumstances some light will be emitted, but generally speaking the two black holes merge, scream out their gravitational waves, and become a single, somewhat larger black hole… and all this without a photon emitted.
So no light is produced directly. But indirectly, it turns out things can be different.
Artwork depicting two black holes orbiting each other, shortly before they merge and blast out gravitational waves. ...