You mean people who are fully blind are getting kills? Simply timing and spacial awareness. It's reasonable to expect someone to step out of an entrance almost immediately after a flash goes off to capitalise on the full duration of blindness. The trick is to not panic and just time the shot for when they run out, providing you haven't moved your crosshair that is.
Another way, say something like banana on inferno. These guys know their spray patterns so well that they don't need the visual element to make it with, it's all muscle memory so they can give a fully accurate spray down a narrow corridor to some effect.