Question

Why would an isolated black hole in space be difficult to detect?

Answer 1

Since not even light can escape a black hole, they are only visible due to their effect on other celestial bodies.

Explanation:

We don't see any black hole. We see quasars. We see the effect gravity lenses (when something like a galaxy passes behind a black hole the light from that galaxy is distorted by the gravity of the black hole). So if a black hole was isolated by itself with nothing for it's gravity to affect we wouldn't see it.

I tried to add a link to a simulation of gravity lensing, but it didn't want to work so now it is posted in comments below.