That sounds like "duct seal". It is used to seal transitions in Heat/Cooling ductwork.It was covered with some sort of rubbery substance I presume as a way to treat the wound. Somehow water got in it and rotted the area underneath. I discovered this afterwards when I pulled back and removed the latex-like coating.
I've heard of people using it, but have been hesitant to try it myself.
I think I'll stick to cut paste and putty!!