Is that black dragon the same one from the start of the game and if it is I'm probably going to have to kill it right?

answering your question would require posting a spoiler. too bad we're not in the spoilers forum.....

Oh no, it's not that black dragon. It's that other black dragon. The one on the left.

Why am I having deja vu here? Is it because vague questions that assume we all know what you're looking at are so common? Or is this a repost?

It's not a secret. That is Alduin, the world eater.