Black markets have a number of negative effects. First, they deprive governments of tax revenue. Black market transactions aren't taxed or recorded as income since they typically take place using cash without a paper trail.
Additionally, black markets tend to take a business away from law-abiding entrepreneurs. For example, if someone performs a service without the necessary certification, they are undermining the regulatory system and taking a business away from operators who are following the law. This is part of the controversy around Airbnb (ABNB -0.11%) and other home-sharing platforms, which some critics call illegal hotels.
Finally, black markets often fund criminals. Money from selling illegal drugs or weapons can go to more nefarious purposes, funding higher-level organized crime, for example, and further entrenching crime.