I was referring to lower cost (but legal) services. For example, living in a cheaper apartment complex generally is riskier, because there tends to be more crime in the area than there would be in more expensive apartments. This point (which I made in my third paragraph) is separate from the point I was making about black markets.

In the case of unlicensed cabs, they are clearly in the "black market" category. As with all black markets, they aren't necessarily cheaper than their legal equivalents, but nonetheless they are there due to unmet demand in the market.