Sps such as montipora can also start browning, which is a sign of insufficient nutrients and/or lighting. I usually keep my newest sps frags in the mid section of my tank and slowly acclimate them towards the top, giving the corals more and more lighting gradually.
Yes, darkness can cause bleaching, but usually you need really low light levels (i.e., keeping the coral in the dark) to get that reaction. Unless the coral is completely shaded, or fell behind a rock, I wouldn't expect low-light bleaching under those lights.
Is it possible to see a photo? Sometimes it can be difficult to tell the difference between bleaching and tissue death.