Someone told me that, under California law, blackpowder muzzleloading guns are considered to be antiques.

Not necessarily. There are different definitions of "antique firearm" for different situations. For example, the definition of an "antique firearm" in the context of transferring firearms is different than the one for certain prohibited weapons. Look at Penal Code section 16170 and see if your particular gun falls under any of those definitions. It is very possible for a blackpowder muzzle-loader to be an "antique firearm" which does not need to be transferred through a licensed dealer, but not all are, so you better make sure first. If you have a specific question about a specific gun there is a lot of information on my firm's website about firearms law in California, and if the answer is not there you can call for a consultation. Michel & Associates, PC (562) 216-4444