Only in states that require registration. In civilized states, that don't require registration, those guns are no more "black market" than your unregistered tomatoes.

Technically, many "black market" guns are "grey market" guns. Agorists make a distinction between items prohibited by the state (black market) and items the state issues licenses for. Doing otherwise legal things without complying with licensing requirements is considered "grey market" activity.

Usually, black market is the market in goods that would otherwise be bearing a heavy tax.
An example would have been salt in India and tobacco around the world.
(BTW, nicotine kills all [as in every] virus known to exist. Maybe this is why they tax and discourage the use of nicotine products like tobacco, tomatoes and eggplants.)