My only concern would be that by eliminating the bottom layer of tank supports it would be a little less stable. If you could tie in the cinder blocks to the existing tank stand beneath it in some way that would help. If you did just the back 2 legs you should be able to conceal it pretty easy. I've found that the pressure treated 2x4's are much straighter & higher quality than regular construction grade, They're a bit more expensive, but it's worth it.
You could paint the blocks & wooden framework to match the stand beneath it. That would give it less of an industrial look.
Tolak