This phrase originated exactly as we all think it did. Star Wars, the brainchild of George Lucas, introduced society at large to the dark side through the character of Darth Vader. The phrase did appear in a 1650 publication from theologian Jacob Bauthumley but stayed out of use until Star Wars. Read all about it here . The phrase also means just what we use it for, the evil side of a personality or society.

George Bernard Shaw seems to have been the first to use this phrase in a figurative sense. And it seems that the figurative meaning is the same as the literal. Taking a shot in the dark is a hopeful attempt at success. ( source )

Dark Horse is a term that was born in horse racing circles.  Click here for the origins of the term and for uses of it. (And watch the video - fun!) These days it means anyone who comes up from behind or out of nowhere and takes first place, so to speak. For example, my current job position. I was called the dark horse by getting the job offer. They had already done several interviews and were pretty much at the end of people and ready to make a decision when my application came in and they decided to do one more interview. I got the job. *wink* Came in like a dark horse, ha!