I am using GIMP 2.6 on Ubuntu 11.10 and wish to chage GIMP's Interface to black. How do I do that.
There seems to be an answer to your question on GIMP.org.
You can do this by choosing an appropriate GTK+ theme. There are a few in Ubuntu already, and a lot more at e.g. http://gnome-look.org/?xcontentmode=100
Note that some theme creators are more interested in the looks than in usability, so you might encounter problem when trying to use GIMP with these themes.
You can install themes for use in GIMP only without affecting your desktop or other applications, this is described in the GIMP user manual: http://docs.gimp.org/2.8/en/gimp-pimping.html#gimp-prefs-theme