These weapons are already at level 6 and can not be upgraded.
"Sloth Demon" is an anagram for Tom Sheldon. In the third Agency mission (" The White Tiger"), it's revealed that the Black Market dealer is Tom Sheldon. Spoiler warning: Significant plot details follow. You can not get extracted while you are in any heat level or standing in a military base unless you have 100% completion on the base. You can only call the black market dealer when you stand on flat ground, swim at the surface of open water and when nothing is blocking helicopter access above you (bridges, buildings.). The black market dealer always uses an H-62 Quapaw helicopter.
The beacon is activated by pressing 4 then E (PC) ↓ then Y (Xbox) and ↓ then ∆ (PS3). The black market is unlocked for use by the player at the end of the second storyline mission Casino Bust and can be called by using a beacon. It can also be used for fast long-distance travel ( Extractions). The black market dealer, initially known as Sloth Demon, is the one who sells weapons and vehicles to Rico.