I would have to assume what you're seeing is not soot, as left by excess fuel, but oil , probably from a worn valve guide.
The way these engines are designed, if any cylinders get too much fuel, it will be the center ones. That's what is making me think you're seeing an oil problem.
You've obviously done a compression test, your numbers are even, so it should not be in the rings. That leaves valve guides.
Devin
