Its not hard to get at: in times of plenty people don't need to look hard for ordinary things to buy, but in times of crisis (such as with certain states) a bonus is added to these items. So food would attract a high price on the BM above the fixed price (and low quantity) available locally. So the BM here acts as a stopgap, but at a much higher selling price because people are desperate for items beyond legal stock levels.
'Always illegal' is whatever is illegal in that system due to its gov. These should always command a high price.
I still don't get the distinction you're trying to make, or rather, what you're trying to suggest that isn't already in the game.
Famine puts a premium on food prices.
Outbreak puts a premium on medicines.
These also carry through to the black market.
Are you suggesting that in Famine, the open market should remain "cheap" while not-illegal, not-stolen goods are still able to be sold on the BM for big profit? That would be (in terms of game mechanics) pretty bizzarre.
So, no, I still don't really get what you're going for here. Price bumps due to scarcity from stateful crisis are already in the game, and it sounds like you're thinking "make those spikes BM only".
However, once an item becomes controlled, that's when it's price spikes
if wealth is available. There's plenty of RL examples of that, but I don't want to start talking global-politics and economics here to provide examples.
Scarcity of an item on it's own doesn't necessitate (black market) value. Regulation and control of an item does.[1]
A legal gun is more expensive then a illegal gun. The latter has no re-sale value
Same for a painting. A stolen painting has less value.
Even tru for weed here in NL. I can buy that cheaper in less legal ways than in the coffeeshop (no sales taxes)
That's far from a universal truth... but I also don't want to start poking at RL examples to try and explain why that would be happening, because that gets fraught fast as noted above.
[1] But neither does it follow that a regulated item will be valued more than an unregulated one, as other factors can affect that.