Black holes are black. No light gets out of them, so they can't even reflect light. What we see in the centers of galaxies is the infalling matter, which is raised to high energies by its fall and radiates before it hits the event horizon. Also the BH is surrounded by perhaps hundreds of stars in orbit, as shown in the time lapse pictures marcus linke to.
So what could you see of a black hole out between the galaxies, away from any matter that could signal its presence? There wouldn't be anything to see. There might be black holes out there, but we'd never know it. Even their gravity wouldn't show since they'd be so far from visible matter.