I'd like to think I'm pretty hygienic for your average Gamer™️ but there's just nothing preventing these bad boys from getting dirty in the back. I've been obsessively washing my hands before and after playing, scrubbing them down after every session, etc. but this is typically how they look after a solid few hours of CoD.
Honestly I don't know what Sony were thinking with this color scheme other than "Haha, they'll flock like sheep for the first all black DualSense controller!"