Negation. accusative changes to genitive in negated sentence.
Masz czarne skarpety- nie masz czarnych skarpet.
"You don't have black socks", should be right, you can write " You do not have black socks.", or with "Don't". This is the some.
Well, there was plural used in the sentence, and Genitive plural (needed by "nie masz") is "skarpet".
"skarpety" could be Genitive singular (Nie masz czarnej skarpety), one sock.
And of course "skarpety" is Nominative plural, and also Accusative plural.
The Polish sentence doesn't specify that it's about wearing socks. Nie masz czarnych skarpet either means that you don't own any, or you just don't have any with you at the moment.