Some people are more concerned with their health than others, the black market is for those who can't afford to be concerned with their health.

In Michigan, black market was cheaper for a while as dispensaries seemed to focus on high-end product, but prices have significantly reduced over the last few years (I'm guessing due to larger scale of production and more sophisticated business models and logistics). An ounce can now be legally obtained for ~$100-150 and black market product is only available in jurisdictions that haven't permitted building dispensaries - and most of that is legally obtained and illegally resold rather than smuggled or illicitly grown.

I've heard but not directly experienced black market, that their primary customers are either underage users or people who want to convenience of delivery and to keep their contact information away from the watchful eye of the government, and that the people who provide the service typically buy form legal dispensaries and then upcharge dramatically to cover the hassle.