Bigbore,
The rule says:
"In any hunt, including any-weapon seasons, it is unlawful to pursue or kill big game animals with any rimfire rifle, rimfire handgun, or muzzleloading handgun, except mountain lion."
So, yes, you can come and whack a mountain lion with a black powder revolver. It is not a special black powder hunt. It is the regular mountain lion season.
Most lions are killed at very short ranges when they are treed by hounds or come to a call. If you plan to hunt with a guide, it is best to check with them because most of them require a larger cal centerfire weapon to minimize the chance of their hounds getting chewed up before the lion dies.
Sixgun