Accepting something without inspecting it. Agree to something without reviewing it first. The phrase suggests that someone’s desire for something or someone is so strong that they need not review nor examine it to want it. A good example is during the late 19th century cowboys ordered ‘mail order brides’ to marry because unmarried females were so rare in the Old West.
“When I told him I had a 1950 Chevy, he wanted to buy it sight unseen.”