No the wording is fine. I like it, although I wonder if the life gain is really appropriate. Life gain in black tends only to be accompanied by damage, a la Drain Life.
The flavor is that you're draining the graveyard instead of life. It's similar to Psychic Drain which drained the library. I figure black can have life gain as long as it's draining
something. It does need a better name to convey the flavor.