I also would be interested to see how a doctor can ethically give you a prescription for a substance that does absolutely nothing to benefit you.
This is not what is meant by legalization. There would not be a black market because doctors would not prescribe these drugs, you would be able to buy them at your local, licensed convenience store (with proof of ID).
Is there a black market for alcohol or caffeine? If there is it's so tiny that somehow I've never encountered or heard about it. The same would be true for heroin, cocaine, etc.