Black market marijuana costs one-third as much as marijuana sold in legal (I-502) stores, according to an article appearing in today's Seattle Times. "State Liquor Control Board (LCB) Director Rick Garza acknowledged a black market for marijuana likely will remain even after a system overhaul. Garza assumes about 25 percent of the state's current black market is considered to be people under 21 who aren't allowed to buy recreational marijuana legally."
Black market marijuana is largely shared or sold to high school students by their friends. Results from the 2012 Seattle Public School Youth Risk Behavior Survey show that almost 70% of high school students who use marijuana get it from friends.
Black market marijuana in Washington is largely grown in Washington. So much is grown locally, Washington weed is sold throughout the United States.
To eliminate the black market, Washington and local jurisdictions can invest in substance use prevention to reduce demand and enforce laws to reduce supply. Changing the medical marijuana system will help reduce a significant portion of the supply to underage users, but not as much as investing in methods for preventing youth marijuana access and use for which coalitions like Prevention WINS have been advocating for long before state voters approved I-502.