Ribbon eels are very hard to keep. They usually don't eat and need to be kept in escape proof tanks. As for being reef safe they will not bother fish, unless they're very small and can fit in their mouths, and will usually leave alone most inverts, though they will get ghost shrimp and small inverts.
The Black color phase is the Juv color of the eel and soon, if it lives long enough, will turn into the blue color of the adult.
Unless you see the eel eating, and can provide it with a very secure tank, it's best to pass on all ribbon eels. Also don't go by the LFS telling you that they have had to eel for weeks and it must be eating some thing or else it would have died by now. Most eels can go months without eating, unlike most fish, and still look fine.