It's supply and demand. Black market prices are for sure lower than recreational, and depending on a number of factors, lower than medical, but black market is a whole other subject....
Medical and recreational are separated. The same dispensary grows it, but they have so many plants for medical and so many for rec.
More and more dispensaries are starting rec sales but there are far more that are still medical only....
So rec is more expensive because of lower supply and high demand. As long as they can sell the majority of their recreational supply at a high price, they will.
Like I said, some now sell quarters for the same price medical and recreational, but its the lower quality places and their lower quality bud.
If you want the high quality shit, it's from the smaller dispensaries, which already have higher prices in the first place for medical. But now also have an even higher demand on their even more limited supply.