[vecter]

That doesn't follow from the fact that black holes are "black". For example, black holes can emit a ton of radiation from accretion disk as matter accelerates and falls inwards, radiating huge amounts of energy [0]. Two naked black holes merging probably wouldn't emit much (if any) EM radiation, but if they had very active accretion disks, then it's certainly possible there would be a ton of EM activity.

[0] https://en.wikipedia.org/wiki/Accretion_disk

[krylon]

Yes. I had not thought of that. But if either of the black holes (or both) had an accretion disk at the time of the collision, they would have been "visible" before.

I am not sure, though, if regular (stellar-mass) black holes with an accretion disk emit enough EM radiation to be visible at such distances, or if it would become lost among the radiation emitted by the rest of the galaxy.

(A merger between two supermassive black holes with active accretion disks must be a spectacular sight even from a safe distance.)