Solid black on most of my tanks. I really like the dark background...everything seems to stand out more without any distractions. I've got picture backgrounds on a couple of my older smaller tanks.
I have solid blue on both my larger tanks. I have never tried the black but I think I will. It makes perfect sense.
I have never cared for the fake rock/fish/plant background and the fake coral ones on a freshwater tank drives me nuts.
I'm another die-hard black background fan. I use black paint on all my freshwater tanks. Another color that looks surprisingly good and is very natural looking, is dark green. Try it sometime.
Saltwater tanks look good with either blue, black or sea-green.