I think it's ferric (Iron III) phosphate, which is black, and organics (cyanobacteria). Flush it like Bill suggests. Soak the stuff in sodium hydroxide (Drain-o) and you've got some pricy salt water aquarium filter material, ferric oxide, which is redish. Except I expect the contaminants would kill the fish
Iron III loves phosphates and so does cyanobacteria. You'll see the pink cyano causing problems in places with light, like pools, aquariums, toilts... The black cyano shows up where there's no light, like your exhaust system.
Flush, flush, flush
