A Pig in a Poke
Buying a pig in a poke, colloquially, is to buy something risky without examining it beforehand. The phrase arose in the middle ages, when confidence tricksters would offer to sell a pig in a sack - but when the customer opened the sack, they’d find they’d bought a nice juicy cat. Or rabbit.
These days, the pig in the poke is more likely to be a nice juicy “risk-free” yield on a structured deposit.