I thought the Dark Ages were Dark because they lacked any significant literary works. Everyone was so busy simply surviving and dealing with the fall of Rome, and the invasion of the Goths, Vandals, Franks, etc., that they didn't have time to do nearly as much writing as previous cultures had been able to do. They're "dark" because we don't know as much about them due to the lack of extant literature from that period. We don't really call them the Dark Ages any more; now it's the Early Middle Ages.
I suppose I'd call them dark too though, if I had to live through the fall of Rome, the invasions of various barbarian tribes, and the reshaping of Europe's political & economic landscape.