If it's cement, it may affect your pH and hardness somewhat, but depending on what you have in your tank, and how big/often your water changes are, that may not be an issue.
No idea on the black stripe. If it was used with building/roofs, it may be some sort of residue from a tar-like compound. The good thing about it kicking around in the ocean for a while is that anything likely to leach out hopefully has already done so.
Sorry, I don't have anything more helpful for you, maybe someone else on here can think of something.