Black Markets always exist under the normal economy, making a potential harm to free market. So I agree on this comment that black markets of a certain scale would badly mess the normal economy and even more noticeable consequences.
Depending on what exactly is dealing in black market, different consequences would occur to society. For example, if drugs are dealing widely in the black market, huge profit would be received by the drug dealer but it would cause violence and chaos among the whole society. Therefore trading in the black market definitely work against business ethic, which even reach beyond the boundary of legal issue.
But I think there is still a considerable problem even if the black market can be completely shut down by official power. Even U.S. and European police are now tracing and targeting the black market trading, demand for products in black-market would still exist. The police representing legal power have right and responsibility to stop this black market trading behaviour in order to prevent black market trading and protect the normal economic environment, however, they are unable to prevent the demand and willingness to sell in black market.
If tradingchannels are shut down by official power, people in this profit-motivated underground market would possibly seek for other extreme ways to make the deals, which would be very dangerous and unpredictable consequence.