It could, but I think it would lead to problems. Carbon isn't very dense, so it tends to get stirred up easily. I think it would also absorb a lot of nasty stuff and start leaching it back into the tank eventually.
If you're just looking for black substrate, there are options out there that are comparably priced to carbon. Tahitian moon sand comes in black. I believe EcoComplete (I think that's the right name) also comes in black.