Some manzanita DW isn't going to effect your water perimeters or color one bit. I've used it in all my tanks and it has absolutely 0 effect on water color and perimeters. I don't use it anymore though. I liked the look but it was a royal pain in the ass getting the substrate vacuumed around it while doing water changes so I discontinued using it.
As far as for the background w/o moving the tank there are ways. I had a 95g against my wall that I didn't want to drain to pull out and spray. I went to the dollar store and snagged two pieces of solid black poster board. Razor cut them to the exact size, and I just pulled it tight around the edges and taped it. I wasn't sure how this was going to turn out but it literally looked just as normal as my other 125g that had the back sprayed black. Worked like a charm.