Eclipses are possible in KSP because the Mun, like the Earth's moon, isn't in an offset orbit. Because of graphic engine limitations, it does not get dark (unless having the Scatterer mod). However, solar panels will say that they are blocked by the planet or moon in the way and not work. Below are some pictures of a Kerbolar eclipse, as viewed from Kerbin and LKO (Low Kerbin Orbit).
A full Kerbolar eclipse can be observed while landed at KSC starting after game time Year 1, Day 31 - 5:04:43, with its highest point at 5:12:40, and ending at 5:22:04. Not sure of other occurrences or locations, but this one at KSC is confirmed. The moon makes a sort of cycle with Kerbol every 6 days. So, after every 6 days, a Kerbolar Eclipse can happen. There are also some sources claiming there is an Kerbolar Eclipse every 28 days (this eclipse doesn't have to happen at the KSC though.)