Not a lot of Black Market gun dealers on the Darkweb these days, legit ones anyways, I wish I knew vendor that sold firearms.
A leit market to buy a weapon, would be from a local licensed FFL dealer, at a firearms store where people normally buy firearms.
Or you could get a "street gun" from someone you barely trust.
For home defense just get a legal gun, the biggest thing you gotta worry about is the cops taking you gun, for evidence, because you gave someone a bullet wound through their ass meat.
If you need to accomplish 'You Know What' just buy a gun legally accomplish 'You Know What' with the 9mm, or 22 because its quieter, or a 38 so you don't leave bullet casings, then take the gun apart and throw away the gun parts in different places of your town, city, state, country.
If you are unfortunate and live in a country that doesn't sells firearms, just get a street gun, some drug dealers know people who sell guns, good luck with that.
I had fun writing this lol
Thank You.