ATM in the US the black market is actually expanding, it's also changing.
It's not so much about bringing it in from out of country, but running it from State to State. Because suddenly anyone can take a trip to a legal State and take it to an illegal one you see more "normal" people getting caught on trafficking charges. It's becoming a crime of opportunity. Until the Fed changes this will be the case. What we've literally done at this point is to simply move the supply closer and actually made the large seller buy easier.
It's complicated by the inability to move product between States legally. Growers are producing more than the State can use often, and have no where to dump the excess except on the black market. The legal industry is still too new and rapidly expanding to have figured out all the logistics.
I think it will eventually shake out that dispensaries will still carry what will become the new "commercial" bud. Limited numbers of strains that grow uniform and produce uniform buds, taste, etc. but the real market will be processed products like extracts, edibles, vapes etc. This is where the THC/CBD/CBN etc. content can be uniformly controlled and homogenized. which is the goal of large scale agro business. The mass market is all about uniformity of product that appeals to the widest range of people possible.
From there I suspect cannabis to have one of the largest artisan/craft industries around. This type of operation is practically a direct plug and play from these days. Thats where you'll find the top notch buds with great variety etc.
Prices are gonna freefall again once it's federally legal, and the industry can use banks and more importantly take their appropriate tax write offs which they can't now. When they can move it across States the market is gonna get flooded with weed making it a buyers market for a while at least.
The black market for weed will pretty much die out in time, there will always be some form, I mean hell there's still a black market for alcohol but nothing like during prohibition, because in general, people are willing to pay for convenience, ease, familiar and "safe" products.
Sometimes I think the black market discussion tend to think that suddenly there will be no black market, it'll still be there there are plenty of other drugs they sell, and as long as any regulation exists there's someone who will want it but can't get it legally.