Sorry to be a dick. Just a couple thoughts:

GDP calculations are estimates derived from various measures of economic activity, different by industry. In other words, there’s no neat way to split out formal vs black market activity. In any case you would want to include black market activity, since it’s part of the economy.

Some countries have relatively larger informal markets. For example many second and third world countries have ‘honeypot’ problems where much of the economy is centered around the state, providing a natural focal point for criminal activity. These are the stereotypical cases we imagine when we think of ‘the black market,’ but in actuality the vast majority of criminal economic activity occurs in the West (like economic activity generally).