To tell ya true Jo, I dont often candle coturnix. some you can tell and some you cant. but to answer your question, I candle at 10 days. the black or completely dark ones (with the exception of the air cell) are the developing eggs and the ones you KEEP. they are dark because the developing chick prevents the light from passing thru. Coturnix spots make it tough to get a light to shine thru some even if they are clear.
As to how long to brood. For the record 6 weeks. But in reality you can shorten that up alot if your in a warmer climate, or raising them in a barn or shed. I try to keep mine in the house for no more than 2 weeks. they will get stinky with a quickness. The dust or dander they produce is bad too. I then move them to my un heated shop but I have a heat lamp set up on a wafer thermostat. the goal here is to keep them just a wee bit cooler than they want to be, and they will feather faster. Its just nature. Before you just move them out side with no heat at all, Check under their wing. Its the last place to feather out proper.... Holler if I can help, Bill