Posses a gopher.

Find a cliff you can fall off of while digging (in mole form).

Begin digging in mole form and dig off the cliff.
Remember to dig all the way off the cliff.

You should have a pitch black mole now.

I'm sorry there are no screenshots, i couldn't figure out how to get them.
the glitch is reversible by digging and then resurfacing.
This might get patched, so if it doesn't work that's probably it.