Sailfin blennies, Emblemaria pandionis, are small, black, grey or blennies that are great for reef tanks because they don't get big, they don't pick at corals, and they get along with most other fish. 

 

The males are generally jet black and have a huge dorsal fin that looks like a sail when they stick it up, which they do frequently.  Females are generally a lighter color than the males, and do not have as large  of a dorsal fin, but they are neat too. Several males can be kept together in one tank and although they will strut their stuff and challenge one another, they will not inflict any serious injuries.

 

This is the only picture I have for now, and it doesn't show the high sail fin dorsal, so you'll just have to imagine what it looks like.