If substances of known purity and strength were available from trusted sources, people would want something stronger and procure it from un-trustworthy sources.
That assumes that stronger things are available from untrustworthy sources. It would be quite possible to sell undiluted fentanyl under a legalization scheme, though I think I'd require a class or really hefty waiver first.
It also disregards the prevalence of popping pills diverted from our legal pharmaceutical system before the latest crackdown.
As lupinus mentions, this just isn't born out in the real world unless we put stupid levels of taxes on it.
There is plenty of black market sales for the under 21 crowd.
But, correct me if I'm wrong, there's no organized gangs out there shipping alcohol across the country, even smuggling it across borders, distilling it down in Mexico using lead pipes and spiking it with methanol and battery acid to 'give it a bit more kick', right?
We're talking more about the occasional clerk who'll sell without ID to teens that he knows, homeless drunks that will split the 'take' with a teenager willing to fund a couple bottles of something, 21 year olds wanting to be 'cool' with the teenage crowd, etc... The product they're selling is identical to the legal stuff, just diverted.
It's when the black market people get into manufacturing that you get the horrors of non-existent quality control.