Legalizing would _grow_ the black market? How on earth do you figure that?
And you can't compare the results of legal prescription of opioids under the current regime to their legalization and regulated distribution. 
Legal prescriptions lead to abuse because people get addicted and then get cut off from their prescription supply. If they had access to a regulated product, they wouldn't turn to the street.
Yes, they'd still be addicts... but they'd have a lot more options for treatment.
This is pretty obvious stuff.