The word dark has rather "dull" origins. In Middle English, it was derk, and in Old English, it was deorc, both with the same meanings as today (simple words like this rarely experience a lot of semantic change). Before that, we can reconstruct it to the Proto-Germanic root derkaz, which simply meant "without light", and didn't have all those metaphorical extensions we apply to the word today (more on that later). This is from the Proto-Indo-European root dherg, which was the definition of "dull", a pretty obvious connection to make. Curiously, during all this time, dark as an noun (such as in the dark) didn't exist until the thirteenth century, or during Middle English. Figurative uses of darkness to describe evil also became applied in Middle English, this time during the fourteenth century. It's quite possible that the word was used like this in Old English, but there's no way to be sure. Curiously, usage of the word dark has been increasing since the 1970s, so perhaps we're descending into the dark ages once more.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.