I used a blackboard paint,I'll find the exact make soon if you need it.It bubbled a bit once the tank was up to temp,I worried a little,but its given a nice ripple effect on the back
I would get some card the same size of your tank then paint the thin card instead of painting the glass as you might want to change your mind in the future and this will stop you from removing the paint from the glass of your tank? Just something i would do.
I paint mine with ordinary gloss black spray paint. Painting the back of the tank gives a much better look over using a ready made background IMO and if you change your mind later on, you can remove the paint with a razor blade.
Never done it myself but I'd say some form of latex paint used for models etc would be best, it should handle heat change well and stick to glass well, like I say though I've never done it myself, I have used a standard black backing before now but currently have nothing, relying on my wall colour (it's not black!, light blue in one room (CA breeding) and beige in the other (SA with tannins i.e. yellowish water))