Sorry, Trev, I didn't read your original post correctly. Shutting down should produce a list on your screen consisting of:
Shutting down service foo ..... done Shutting down service bar ..... done etc. Runlevel 0 reached or something like that (as you are no doubt aware), in one of the VTs, while other VTs go blank. The blocky display sounds to me like what you get when you switch from a graphic display mode to a text display mode with a buggy graphics system (buggy card and/or buggy driver).
