Cleaning Red Leather

I've got an M235i convertible with red leather. I drive top down most of the time. I noticed yesterday that I am getting an accumulation of black grime on the back seats. It's sooty and sticky; it reminds me of brake dust or the carbon accumulation you see around tail pipes. (I'm now a bit worried about exhaust blowing into the passenger area when top-down, but that's a different concern.)

I went after it with Maguires Gold Class leather cleaner. That took most of it off, but there are still a few shadowed looking spots. Are there better choices to completely remove this?

Cleaning Red Leather