It is not just Houston, but other cities as well. I have heard the following reasons why particular ladies don't see black men:
no attraction to black men and thus it would make for a non-enjoyable experience
black men smell different than white men
black men will try to haggle
the precipitation that a good amount of black men are thugs and/or immature (thank you mtv and bet)
they date black men outside of the hobby
other hobbyist will not see them if they know they see black men
some providers are just straight up racists
Other times you will have providers that will see black men under certain conditions:
If you are above a certain age
Only if they are doing doubles with another provider
Honestly it is what it is, just be upfront with who you are with them and if they don't want to see you move on. Once you are able to see a provider post a review, as having reviews out there will help with screening for future appointments