Gosh, ok read this article:
http://www.aquahobby.com/articles/e_aquascaping.php
Its for planted tanks but you can extrapolate out to how to work with only hardscape(rocks, dead corals, substrate and driftwood)
As for the back ground, Blue looks best with dim lighting too much light will reflect off the back ground and turn everything in your tank a strange lopsided blue. Blue is also only appealing with while substrate and dark fish. If blue is what you are set on indeed a raised LED light would look best as the simmer playing off the blue back ground is very appealing.
Black is my preferred but I haven't seen your oscars, they'd have to pack some color in them else it won't work out with them as your only fish.
Tanks, are easily broken down into different elements, lighting/color, and hardscape/visual direction (fish feed into both of those). Seeing as you have fish already you will want to reverse engineer these elements to maximize your tank. Oscars are large boisterous fish that will pull and push and dig eventually so your probably going to stay way from plants in the long run weather fake or live unless you can anchor them to drift wood or holly rock or something. I personally find convex design to work best with your type of fish as no matter where they are in the tank they will work with the visual direction of your hardscape.
You can build this with larger rocks and smaller pieces of drift wood or one Large labyrinthine drift wood and or pile of coral and or holey rock. What you choose for hard scape will have to reflect the back ground you settle on and the substrate you choose to use. IMHO crushed coral substrate looks best with blue, or a white sand. Drift wood doesn't look as good in this situation so holly rock and or coral in the center of the tank is best or large rocks piled up like this:
seed how on the right they nest together and point out from a central point, you can probably do better I put that tank together in an hour with no pre meditation about it but you get the idea, large slivers of slate, or any rock for that matter will work. The rocks need not be sharp or even pointed rather just oblong. Center the pile in the relative center or back center of the tank. The idea is to create visual direction with the curves of the rock to guide your eye to the center of your tank and then back out its a real good play with positive and negative space. Plus no matter where your fish are or what they are doing they will fit into this element unlike most of the other layouts where fish shape finnage and behavior figure in more prominently.
Anyway there are more ideas I'm sure you have if you understand the principles above and read about aquascaping and aquarium design you'll develop some of your own. Enjoy and its cool to see someone on here doing more than putting large fish in a bare bottom tank.