I prefer the black background just because it tends to be less noticeable than blue. That being said, if you continue on heavily planting your tank, it won't matter what color you have!
By the way, good decision on removing those Amazons. I made the mistake of putting 4 in my 58 gallon and a year later, they were WAY too big for the tank and I removed them, one by one to make room.
Looks like you've got some great progress so far!