Something I'm having a little trouble with is figuring out what, if anything, the PCs need to gain an audience with a merchant prince in order to get.
Ekene-Afa: Many of her items are for sale in the markets, like canoes and raincatchers. Would it be fair to assume that the PCs would need to deal directly with her in order to purchase any of her magic items?
Ifan Talro'a: He has "dozens of merchants" who sell his beasts for him. It seems like one needn't meet with Ifan at all, unless you're a Zhent wanting to buy a flying snake.
Jessamine: I'm guessing her herbs and plants can be purchased at the markets, but you'll need to see her in person to buy any poisons or sanctions.
Jobal: He doesn't seem to sell anything as such, and I can't really see any reason for the PCs to seek him out personally, since the guides advertise their services around the city.
Kwayothé: It seems like one would be able to purchase everything she controls in the markets. No need to see her directly at all (unless you're up for a bit of S&M).
Wakanga: He seems like the only one who does absolutely need to be dealt with directly, since he mostly deals in knowledge, as well as potions and scrolls.
Zhanthi: Most of her stuff should be purchasable through the markets. There doesn't seem to be a need to see her directly either.
Given all that, there doesn't seem to be much point in using the black market. There aren't many things that can't be purchased at standard prices on the open market, and there doesn't appear to be anything you can only get via the black market, so I can't see why anyone would bother trying to track down the black markets, especially since everything will cost at least 3x as much as the standard price.
Am I missing something, or is the black market only useful in situations where you can't buy an item through legitimate means, for whatever reason? (Like if you can't get an audience with the relevant merchant prince or if you've offended said prince and they've blacklisted you.)