This question has one claim in the title and four others in the body
1. The black market in Colorado has increased since legalization
Possible. There are claims many places including here that legalization has created an environment where illegal and quasi-legal growers can grow and export to other states.
But that's unlikely to be correlated with increased black market sales within Colorado, since it's only illegal for teenagers and children, and statewide surveys of youth in Colorado, Washington, Alaska, and Oregon found that there were no significant increases in youth marijuana use post-legalization.
2. Increase in marijuana-related traffic deaths
Contrary to the data. A Drug Policy Alliance study says:
Legalization has not led to more dangerous road conditions, as traffic fatality rates have remained stable in Colorado, Washington, Alaska, and Oregon.
3. Increase in poison control calls
Contrary to the data. According to the Denver Post:
Three years into regulated sales of recreational cannabis, the Retail Marijuana Public Health Advisory Committee says calls to poison control and marijuana-related emergency room visits are down, even though overall consumption of pot remains steady — signs that existing policy and education efforts may be working.
4. Increase in marijuana-related emergency room visits
Partly, but that may be due to increased reporting (decreased hiding of marijuana usage). A recent report from Colorado's Department of Public Safety says:
[...] it is too early to draw any conclusions about the potential effects of marijuana legalization or commercialization on public safety, public health, or youth outcomes, and this may always be difficult due to the lack of historical data.
Furthermore, the information presented here should be interpreted with caution. The decreasing social stigma regarding marijuana use could lead individuals to be more likely to report use on surveys and to health workers in emergency departments and poison control centers, making marijuana use appear to increase when perhaps it has not.
5. Arrests of black and Latino youths for possession have increased (58% and 29%)**
Unlikely. According to the same Drug Policy Alliance study:
Arrests in [all states] and Washington, D.C. for the possession, cultivation and distribution of marijuana have plummeted since voters legalized the adult use of marijuana, although disproportionate enforcement of marijuana crimes against black people continues.
The quote above from the pdf says "all states", but refers to the set of states studied (in which recreational marijuana was legalized). Here's a quote from the summary (also from the DPA) of the study:
Marijuana arrests have plummeted in the states that legalized marijuana, although disproportionate enforcement of marijuana crimes against black people continues.
Specifics about youth arrests has been hard to find since the airspace is dominated by people worried about the white-vs-minority arrest rates. There's a Colorado Department of Public Safety report which finds
The types of filings did change, with an increase in public consumption and offenses within 1,000 feet of schools, and a decrease for minor in possession and offenses around the 16th Street Mall.
(NOTE: not necessarily minors, but the "schools" part is suggestive)