Honestly, most light-skin normal people find dark skin people just unattractive at best, and ugly at worst.
When it comes to sex workers I am guessing this is why there aren't that many black and brown MPAs, because the market is self-correcting; if darker girls don't get picked out of the lineup then they don't make enough money during a shift, and then they wonder why would they work at a job they don't like for low pay (for the average over the course of an 8 hours shift). This problem can extend to dark skinned guys in dating as well, you know unless you are packing a big black cock down there is no reason why a girl would pick you over a normal guy.
Of course there are exceptions to the rule, personally I think Halle Berry and Zendaya are "good-looking" brown women, but even then they are women that I wouldn't even bother asking out on a date because you can easily find an average white woman walking down the street that looks better than them both.
That being said, I am still looking for that dark skin MPA/SP unicorn, just to give it a try, I always keep an open mind in my regular life and it has helped me immensely, I hope for the same in the sex worker world too.