No they don't need any light and yes it's okay to leave them in the dark. They're nocturnal after all and no it won't give them any issue.
No they do not need daylight. Wherever you read this was incorrect or you misunderstood it.
When they say day/night cycle it's not referring to light. It's referring to temperature drop. Day = hotter than night so that's what it means. BP's would do fine without ever having light. The light cycle is only meant to help them with knowing when it's actual day or night and with proper temperature drops you can put them on a different breeding cycle.