Black Blister On Butt What Kind Of Poisonous Plant Would Cause Black Blisters To Appear On The Skin When Brushed Up Against?

What kind of poisonous plant would cause black blisters to appear on the skin when brushed up against? - black blister on butt

My sister has for his education at the marathon, he ran through some weeds and I published the dark bubbles in the knee area. They looked like a bruise. There were about 9 of them, and after 2 weeks in the dark as black crusts can be very itchy and swollen ...

Any idea what causes it? Thanks for the help!