Corals can go for a few days without light. (Think about if you ordered them online, they're in a box for for a day or two.) I left my lights off for three days during the summer because of heat issues, and I didn't have any problems. I was very concerned about my clams, but the same thing with collection and shipping- they were fine. After three days, I don't really know. I've had frags of zoas fall behind rocks/facedown in the sandbed for a few days, maybe a week even, and they've opened back up after giving them a few days. If you can, try to let your corals get some natural sunlight .
Also, it'd be helpful if you could tell us what type of corals you're going to be keeping out of light, and for how long.