cotton and polyester puma sock I washed with some pooped underwear, so I don’t know if this sock looks brownish due to fecal matter.
I used a cold cycle and abundant detergent. Every other undie looks fine.
I don’t know if I should leave the piece to rinse in a cold water bucket with some detergent and wash again.
I doubt it’s poop, but you can always wash your clothes again (remove any foreign material if there is any in your washer). Most likely, your black socks were cheaply dyed and the dye washed out. Same thing happened with my mountain biking gloves, which lost a lot of their yellow dye after I bled into them and washed them.