"black" live rock
 

I have a couple peices of rock that are black compared to all the other rock in the tank. There doesnt seem to be much if any algea on them. why would that be?

There's a sponge-like hitchhiker that I've seen on some of my rocks & other people's as well that is black in colour. Harmless as far as I can tell & just adds to the tank's biodiversity.

Its kinda smooth and encrusting.

The underside of my some of my rocks are black. Specially in areas dug out by the goby. And yup i heard those were sponges and normally die off when exposed to light