This is actually really not true at all, and it's important to be honest about the reality. First, in many places where weed is now legal, black markets have continued to thrive because there are high taxes on legal weed, and thus black market weed is considerably cheaper. Totally fine to argue that this is then a problem with implementation, but it is definitely not correct to say "People don't even bother getting it on the black market anymore where it is legal." - that's just wrong, and it's not hard to Google for lots of articles discussing this.
Also, while I agree with legalization, I would state that I underestimated some of the downsides. A couple years ago I was in downtown Denver, along their 16th St pedestrian mall, and it's not really a nice thing to see tons of people stoned out of their minds walking around like zombies. Also not great when you get in an Uber and your driver seems totally baked.
Can't emphasize enough that I think the alternative (throwing people in jail) is much worse. But I don't think it's honest to minimize the downsides.
Edit: Save you a google search, https://calmatters.org/commentary/2023/06/california-illicit...