system
A chat on subreddit /r/NoStupidQuestions
human
Roads are filthy and caked with the black soot of car exhaust.  If a tire wasn't black it would be black very soon, except in a splotchy and hideous fashion.
gpt
This was one reason why whitewalls were fancy - it showed you (could afford to pay someone who) cleaned your car

Adding carbon to tires also makes them stronger I think, or something like that.