That's a lot, but it's normal. The black will go away when you switch to a good oil and it's broken in. Which oil is a way to start a war, but there's two that I don't use Amsoil, it makes hard carbon, and Lawnboy=Lawnsluge. I personally use Pennzoil Air Cooled only now, and I stay at 32:1 for ease of carrying only one mix.

My new MT-42 is on the second tank with 6570 rpm using a 20x8 BME running slightly rich, so you're doing good. I think they use the same cylinder, so the difference is probably you using a Zinger.