SP100 with steel wool--nothing
Tried acetone and steel wool--nada
MEK with steel wool--nope
Flitz with steel wool--a little progress
Screwdriver to scrape the black stuff--definitely getting somewhere
The brown residue seems stuck for good (maybe a little better after my efforts), but the black streak was pretty effectively removed with the screwdriver. Unfortunately the screwdriver also lightly gouged the ceramic coating--not terrible, but not great either.
Before:
I'll try to post an "after" picture this weekend. I thought I took one but can't find it on my phone.