I was a dark child too, and continue to be now in my late teens. I strongly believe it is just "me". When I was sad, I would write or draw or study whatever topic happened to be on my mind at the moment, i.e. nuclear chemistry or linguistics or Old English, etc. If I could have chosen to have grown up happy and normal while not possessing my interest in these sorts of things, I would have turned it down and stuck with my melancholy.