The slime blackmarket should be a location where you can sell plorts for a higher price. but to get to it you should have to use 2 slime gate keys or have to deal with alot of feral slimes/tarrs. this is one of those locations where the calming bell will help the most.
In the black market you will see that the plort value is increased by 1 third of its original price.
Of course... A black market wouldnt be a black market. if it wasnt accepting illegal slime plorts.
There should be a entire new region you can enter if you enter the black market. you need 1 gate key in total to enter it.
how to enter it:
One gate is located near your ranch. the next gate is for extra coverage for the first gate. and afterr that you have to Run through a path of feral slimes and tarr. Of course the thing is. you could always put a teleporter there once you manage to survive the slimes. the jetpack wont be of use here. because once you open the gates it reveals a cave with a really tight walls. and a tight ceiling. This will make sure that early game players will not reach the slime blackmarket. completly changing the progression of their game. Once you enter the slime blackmarket there should be 2 hallways for 2 new areas. One of them i call "The Rainbow Marsh" a place filled with colorful grass and giant mushrooms. in the rainbow marsh you should be able to find a "emotion slime" its plort is used as a psychidelic drug which makes you feel happy and heightens your sensations. This slimes plorts can only be sold at the slime blackmarket.
Of course i dont have any ideas for any other slimes or the second hallway. though you could tell me what you think should be added to this idea and so on so fourth