they show up a lot more (relatively speaking) during spawning season because many males (even some immature ones) will turn black or at least darker during that season. they will often revert to "normal" pattern after the season or when taken out of the natural spawning environment.
true melanistic gars do show up from time to time. either way, contact John (snookn) about black gars. outside of that, photos show up here and there, but they are rarely for sale--
--solomon