Soot on rear bumper

[astro4travel]

I have a 2000 Solara 4cyl. with about 124,000 miles on it. It uses about a quart of oil a month with a little leaking involved. The past year I have started to notice some black soot on the rear bumper right above the tailpipe. The shop told me if the fuel mixture was running rich that the engine light would come on. No light on. I was wondering if this could possibly be the oil blowby in the engine? Thanks for any replies.