use a latex black paint which can be brushed on....if you decide you no longer want it, it can be scraped off with a razor. If you decide not to paint, you could also use black trash bags taped onto the rear of the tank
What about one of those black backgrounds that you can get online or in the lfs..? I’ve got those on both of my tanks and they’d just peel off if you changes your mind…?
On one tank I sprayed black Plastidip paint, another has a trimmed black garbage bag, and another is black poster board paper. There are many options but as mentioned, black latex paint also works.