Black construction paper that you can buy by the sheet at hobby-art-craft type stores works fine; inexpensive and it will really make a difference. I would get black, it dissappears; blue will be noticeable as the back wall.
I would not mix substrates, it will look artificial. The sand is probably very authentic, and when the plants grow, and with some more wood branches (the more the better for angels, they live among twisted branches and roots) it will be fine.
As the plants grow and with the above, this will be spectacular. Nice work.