i have used spray paint as well as satin finish acrylic latex. the latter was cheaper and easier for me to deal with. latex will bubble if wet for very long, but can be easily touched up.
I agree, get the cheapest spray paint you can find and just put it on in thin coats and it will come out great. If you look at the tank in my signature, that has a black painted background. I think I used 3 cans of 88 cents each spray paint.Paint - spray or brush on - will look fine from the other side of the glass. Paint the outside back (some might paint the inside) and thru the glass it will look glossy, regardless of the paint.
I've used el-cheapo generic and I've used Krylon spray and brush on. All have worked great for long periods of time.
Unless the background is firmly against the glass (no gaps or airpockets), then it may work. Otherwise, it becomes noticeable when you see these gaps or airpockets. Also of the paper or poster board gets wet, it may degrade over time.If your gonna go with a solid black background why not just use black construction paper or poster board, it'd be easier to remove when you change setups around.