Rubbing Alcohol and soft dry cloth.

Adhesive & Gum remover from local Pep Boys, etc...

Also wax area and buff out to lessen effect of painted area being visible that wasn't before.

 

It used to be called Goo-B-Gone but now it's just Goo-Gone. This stuff rules. It is a citris based orange/yellow colored goo remover found in a small, clear bottle at any Wal-Mart/Target/K-Mart. Just apply some to a rag and first kind of soak the black goo. Then, after a minute or so, start wipping away. At first it will almost seem like you are making things worse by speading it around but just keep rotating your rag and it will get easier and easier. Then clean your trunk real well and possibly even wax it like Wizz said. Good luck.