While many claim to block out 100%, and users claim that they provide total darkness, this is something manufacturers can’t truly guarantee. Experts say 100% is a difficult thing to achieve, as light can leak in through the sides, top, and bottom, depending on how the curtains are hung. If you’re buying the curtains from a brick-and-mortar store, hold them up to the artificial light in the store to test out just how much light is being blocked.