That's not at all true of cigarettes. There has been a black market for them for a long time and they're more attractive than ever with the huge tax burden on them now.
Some say if drugs were legal only to those over 21, there wouldn't be a gang-driven black market to deal drugs to those under 21, just like there is no gang-driven black market dealing alcohol and cigarettes to those under 21. Also, they say retailers wouldn't sell to those under 21 since it would jeopardize their legitimate sales.
All well and good, but in those markets, the illegal under 21 group is small (only 6% for cigarettes), so there's not much lost by not selling to them.
The illegal under 21 marijuana crowd is about 30%, and I would estimate that would grow to 50% with adult legalization. So, when half of a market is illegal, that will bring back the gangs.