I got the mixed-color pack, and they are 54% polyester 45% cotton 1% spandex.
Maybe the 70% cotton spec is correct for the black, but I didn’t order those so can’t confirm. The color ones do feel more polyestery than I like but are still OK. Just FYI.
Thanks for letting us know. I passed it on to the team to research.
By the way Woot customer service sent me an email saying I could return these since the description was incorrect, and apologizing for error. I didn’t even ask other than posting the FYI message here! Woot CS is always very good!
I will probably keep them anyway because I knew I was getting a cotton/poly blend, just the blend was different. So I’m OK with that and don’t think I’ll return them; they are good quality socks anyway. Note that the black colors might be 70% as spec’d. I notice the color combo now shows as sold-out, but I don’t know if that’s because they did sell out, or because they removed from the listing to avoid confusion.
Howdy. We emailed all customers after you alerted us. 
I think that sale was ended but it may have relaunched corrected.