pig in a poke

a pig in a poke — кот в мешке
buy/sell a pig in a poke — купить/продать кота в мешке; вслепую
Meaning: a pig in a poke — sth accepted/bought without first seeing it or finding out

Buying clothes by mail is (like) buying a pig in a poke.

For etymology see [let the cat out of the bag]