It'll take some time for supply chains to get sorted out, and different legal growers to establish themselves. But it should take a big chunk out of the black market sales. Why would an individual sell illegally, when they can operate a legit business doing the exact same thing. No legal risks, no worrying about getting caught, actually maintain a credit rating.
Like with alcohol, the convenience of legal storefronts will be enticing too. I think a consumer would rather walk into a shop on their way home from work and pay with debit/credit, than have to meet up with their drug dealer and bring cash.
Isn't that exactly what's going on downtown right now? The illegal pot shops will be much cheaper than legal ones. It's impossible to shut down the illegal drug trade because the war on drugs failed.