Found: three black holes on collision course

[eppur_se_muova]

3. Black holes themselves do not. But they attract material which accelerates into the black hole ...

... and the infalling material emits radiation as it is squeezed into a more compact mass, where atoms and ions collide more frequently with each other, converting kinetic energy into radiation. The material gets so hot even X rays are emitted.

In the absence of surrounding material to fall into the black hole, it would be almost invisible (except for Hawking radiation).

"The light is the shadow's shadow", to quote an overused line from Somtow Sucharitkul's Inquest series of novels.