buy a pig in a poke
To buy something sight unseen (a “poke” is a bag): “The mail-order offer sounded like a bargain, but I didn't want to buy a pig in a poke.”
The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.