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

roger, roger ... Phosphates... like in lawn and farm fertilizer?

one of the problems here in Florida is that kind of run off in the rivers. I talked to a couple guys at the marina and this seems to be a common problem on the river if you don't run the boat periodically.

Although I did not have this problem on my O'day with a universal motor during the seven years I had it here.