be happy black markets work now, for a long time they were the pirates booty dumping ground, and nothing more...
I usually find (I did some narcotics testing reasonably recently):
Sell Battle/personal weapons in war/civil war/civilunrest systems where they are illegal (not dictatorships, as they produce), if bought at 1kbelow galavg (industrial dictatorship), gives about 2.5k profit.
Narcotics, Buy in low pop agri/industrialdictatorship/anarchy in a boom state, Sell in high pop extraction no state non dictator/anarchy Delain. I found selling in a boom state actually seemed to lower the price, but YMMV.
imp slaves, buy in Torval, sell inAisling/delaine/Antal.
That said, I came across what looked like a bug where antal dictatorships had their black market closed in an Exploited, not controlled system where there Should be 10%illegal good price increase. Narcotics were illegal there too, which they usually aren't. Hard to take advantage of a 10% price bump if the market is closed!