It sounds like you have an addon that is breaking nUI when you first log in.
For starters, nUI does have a big black bar at the bottom... it is a "viewport" and is used to keep the character centered in your screen above the dashboard and so none of the game is hidden behind the dashboard... so that is normal.
But... it should be hidden behind the dashboard which it seems is not loading correctly.
I would suggest you try doing a clean installation of nUI as per these instructions and see if that doesn't fix it...
http://www.wowinterface.com/forums/s...ad.php?t=37330