I have been accused of playing music that's a little dark (although it was framed in different words). In my case, it was because I was playing modal music which is unfamiliar to most people's ears. A lot of the creepy-sounding music that I have heard organists play has been improvisations on chant or compositions based on the modes. Not all of them are creepy, of course, but it will depend on the mode and the composer. Actually, these days almost anything in the minor key will be deemed to be dark, sad, or mysterious.