A light swirl wear pattern on the backplate is normal, as the rod can slide back on the crank pin during climbs and vertical manuvers.
Check the rear ball bearing for "slop" or "play". The bearing should have no noticeable play in it. I just put new bearings in an Evo that exhibited similar symptoms. Mine had gotten to the point where the bottom of the rod had rubbed the bottom of the inside of the case and that's probably where some of my black exh. oil was comming from.
See if you can wiggle the prop fore and aft while looking at the rear bearing.
Black exh. oil can also sometimes come from the muffler "fretting" against the exh. stack, and that's nothing to worry about really, just check the tightness of the muff. screws. It also might just be comming from the flat aluminum baffle plate inside the muffler. Take the muffler apart and remove the baffle and see if the black oil problem stops.