Doesn't it also go by the name of black butyl (same thing?)... when I'm scraping off a goopy mess I first try a small PLASTIC paint scraper - doesn't gouge and scratch as much, then a flat razor blade on windows. A roll of those disposable Blue shop towels don't fall apart like the paper ones.
Cool, I'm about to do repair my trunk "Trunk Pan (and floor pan ) Repair for the Non-Fabricator" and well it looks like someone used this to cover some rust (possibly small holes). The trunk itself is pretty solid (minor rust and a few pin holes) but the black goo was used in what i believe is the wheel well. Anyway, I'll try to post some pics later. Thanks.