I hope I don't run afoul of the moderators, but on another ML forum, that I won't identify, the same question was posed a day or two ago. Several knowledgeable people said it's produced by the burning of Sulphur when atmospheric moisture is optimal for its creation. Sometimes it is seen in the pan, other times near the muzzle. I've never seen it; I'm too busy remembering to complete the sequence of powder, patch, ball, cap.