Hacker News Reader

As someone who hasn't tried Google+:

Is it implemented as a UI of Gmail, or is it its own site?

Own site. Vaguely similar to Facebook, but less "formish" and more interactive. Black bar at top.

I now have the black bar at the top, too. But I am not using Google+. I wonder if the black bar is getting rolled out to everyone.

It is. It will be on all Google's properties. You won't see anything about Google+ on it until you get an invite or it goes public, though.

No black bar on Youtube