That is human nature. The black market is of course illegal. People who buy and sell money outside the official channels are breaking the law and yours truly deprecates, though understands, law breaking in this respect. Of course it is not just the illegality of the practice that is the main issue. The exchange of money outside official channels also means the economy suffers.
Government is deprived of revenue and the much needed foreign currency.
A great deal is being lost as a result. That is why I am surprised the Zimbabwe government is, paradoxically, doing everything possible to make the black market flourish. Or it is doing little to counteract the effects of the illegal practice. When the economy is underperforming as much as the Zimbabwe economy is, then all sorts of problems tend to crop up and no spin doctor, however high their public profile, can overcome them. The thing to do is to manage the economy better and to avoid the sorts of things that undermine confidence in the economy.