They know. Because they understand the concept of darkness from those ppl who were not born blind.

Fun fact: blind people don't see darkness... They just see nothing. It's unexplainable for us people with eyes.

They r types of blindness but if u r talking abt total blindness they see ntg its neither black nor white

Bruhhh they always imaging not just closed thier eyes uk wt im sayin they do see light as well as darkness but they dont differntiate it. Its all in their imagination.

you can't know something u haven't seen before.