Why would an isolated black hole in space be difficult to detect?
Since not even light can escape a black hole, they are only visible due to their effect on other celestial bodies.
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.