Welcome to the forum!
If the blues are in that 10-20lb range, I'd seriously doubt anything else exists in any appreciable numbers.
15lb blue can eat about any BG or other sunfish it wants. If there's any structure present to allow reproduction I would think you'd see some in varying sizes. The blues are top-end predators and will control anything you try to do.
Might want to consider removing 3-4 then stock some adult BG early enough to see if they can get off a spawn. Personally I would stock 100 or so fingerlings at the same time, some may make it through.
Short of removing them all and starting over, that would be one possible approach. Others may have better advice.
