The same reason why brands are desirable in legal markets. Mainly for quality control and the ability for the user to ask for a specific pill that they enjoyed

The same reason why brands are desirable in legal markets. Mainly for quality control and the ability for the user to ask for a specific pill that they enjoyed or heard good things about. That they use well known brands is a side effect of it a being a black market. Why develop a brand of your own that could be taken away at any moment? Reuse well known brands so people will remember them.