Could you maybe use a sticky back vinyl?
Most DIY shops sell them (focus, home base, etc) you could get a plain black or blue one.
Just make sure you smooth it on slow and steady to avoid air bubbles.
If you do still get air bubbles and can't move the bubble to the edge you can prick it carefully with a pin and then smooth it down.
smear the back of the tank with baby oil then smooth on a black plastic bin bag. it does the same job and if you decide in the future that you want the tank clear again you can just peel it off
Generally I use Krylon Fusion to pain all my tanks and plumbing.
If you aren't able to spray indoors (I usually do. Just cover for overspray and open all the windows), then you can really go with just about any paint. If you can, look for one made for either glass, plastic, or appliances.
smear the back of the tank with baby oil then smooth on a black plastic bin bag. it does the same job and if you decide in the future that you want the tank clear again you can just peel it off