From a California viewer who ran afoul of poison oak vines. The cut vines deposited blobs of urushiol oil on his white hoodie.
Each blob of oil then got on into his skin. He got a big rash all over the place, but noticed that the 3 spots were where he got the worst rash.
“Here’s what it looks like when you get poison oak sap on yourself and don’t notice. The sap become black when it oxidized, and left irremovable stains on my clothes. The stains line up exactly with where I got my rash about a week after exposure.
I was out around San Diego, clearing some brush to make a few rock climbing routes accessible. I noticed those black stains on that same day that, but back then didn’t know what to make of them. If I had known about this earlier I probably would have taken a massive tecnu bath that same day and the rash would have become a lot less bad…”