I am not sure what black substance is at issue. I do know that the OEM black rubber damper mount that holds the fuel pump will dissolve in ethanol added fuels. When new it feels like a tire. When damaged it feels soft like a marshmallow. When destroyed it is gooey and sticks to your finger when you poke it. Bob's BMW has commissioned aftermarket alcohol resistant dampers, and has them available. I am not sure who else might also have the aftermarket part.
Hi Paul,
I dismantled the pump clip assembly. The damper was in perfect condition. This is some type of plastic jammed between the metal housing that holds the pump and the left side of the tank. I've never read anything about this being inside of the tank, but it is clearly there on both the front and rear of the pump housing metal assembly. My initial thought was that it was there for structural support so the pump housing doesn't bend downward and rub against the inner left side of the tank.