They’re very dark, sure, but they aren’t black. They glow, slightly, giving off light across the whole spectrum, including visible light.
This radiation is called “Hawking radiation”, after the former Lucasian Professor of Mathematics at Cambridge University Stephen Hawking, who first proposed its existence. Because they are constantly giving this off, and therefore losing mass, black holes will eventually evaporate altogether if they don’t have another source of mass to sustain them; for example, interstellar gas or light.
Smaller black holes are expected to emit radiation faster compared to their mass than larger ones, so if – as some theories predict – the Large Hadron Collider creates minuscule holes through particle collisions, they will evaporate almost immediately. Scientists would then be able to observe their decay through the radiation.
To know more: http://materialsphysics.alliedacademies.com/