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.)
