Either valve seals or rings. have you checked the spark plug for oil fouling? Either way, about the only thing that can be done without rebuilding/tearing down the engine is trying some of that "engine restorer" that claims to revitalize seals and restore compression. I forget the actual name of it, but you've probably seen it on TV.
Might also try some zmax. It works wonders cleaning out the gunk on old engines, that may clean off some valve opening debris that is messing up your combustion, but overall nothing that can fix blue smoke.