>if legal marijuana is priced low enough to make black market dealers less common,
It never is. The overhead from running a lawful business is way higher even before you start accounting for all the weed specific cost of compliance stuff.
That's not the case here in Washington state: legal weed is significantly cheaper than the black-market ever was, unless you want some high-end specialty bud you most likely couldn't have gotten at all back then.
I live in MI and weed is really, really cheap here. I don't smoke, but I partake in edibles (I have MS and nerve pain that meds can't do much about). I can get 2000mg of edibles for 40 bucks. And that's without price comparing: That's just going to the closest dispensary near my house. And lots of places do penny/free joints with a very low/no minimum purchase.
We've had legal marijuana since 2018 up here in Canada, and from the statistics it looks like the market has almost caught up. The closest comparison I can think of is piracy and beginning of streaming wars (like Spotify and Netflix). Sure piracy is free, but a significant chunk of people started subbing for the services because of the convenience. If you'll only save about $5 per purchase, but have to get cash, arrange everything and etc., that might be just enough friction for you to just go to one of the billion stores nearby.
A conspicuous absence of black market for other herbal products such as tea and cilantro indicates that the edge of the black market over the lawful businesses isn't that great, if extant at all.
I think those are just not great examples. There are plenty of examples of black markets for legal things, including food stuff.
Example: instead of buying meat at a formal (as in, legal business that pays taxes, has the proper sanitary reviews, etc) butcher shop, its buying from a small producer who kills their own animals, or from some shady figure who kills other producer’s animals.
Another example is alcohol and cigarettes that are bought at duty free shops by mules and then sold tax free in street markets.
The edge of the black market over lawful business ranges from small to huge depending mostly on how regulated the lawful business is and the costs of a physical location (think street sales of snacks right across from a shop that also sells snacks but also has to pay rent, utilities, taxes, etc).
Or, to take an even nearer example: consider alcohol. It's heavily regulated and taxed in the US, but what little bootlegging exists does so mostly as a curiosity, not as a way of getting cheaper booze.
It varies from state to state, but legal prices are comparable to black market prices in a lot of places. And the selection available in legal markets is beyond compare.
It never is. The overhead from running a lawful business is way higher even before you start accounting for all the weed specific cost of compliance stuff.