Sounds like you broke her in according to the manual. Good. The question of fuel/oil type and amount is a good one. Also, how about prop size and max rpm? Info on those all those things should allow everyone to come up with the best comments. And make sure all of your engine screws and the muffler connections are properly tightened. Sometimes that dark color can be metal from a loose muffler connection.

As a general comment based on about 8 OS 4-stokes and 6 Saitos, I don't like to see a dark or black exhaust and generally find that if I do, I'm running a bit too lean. That's using 20% total oil with 3% castor. So see if you can give us those #'s.