Let's go through it again then shall we?
Racism is the belief in the superiority of one race over another, which often results in discrimination and prejudice towards people based on their race or ethnicity.
So what does that mean? I bet to you it only means the latter part of the sentence. However, in reality, it means the beginning part of the sentence and the latter part is the result.
Now let's look at discrimination
the unjust or prejudicial treatment of different categories of people or things, especially on the grounds of race, age, or sex.
So while one and the other go hand in hand, discrimination - which is what the act of No Blacks in an ad is - can be for other reasons outside of racism. Racism is based on the belief of superiority. Discrimination is often the result.
So now that we have gotten that out of the way. And we recognize a woman's right to choose whom she is sexual with, we are automatically granted a certain annuity in regards to discrimination within a business setting.
Why? Because if it were 100% legal to buy and sell sex, and you took this case through to the highest court to sue on the basis of discrimination, she would still win. According to the the Human Rights Charter and the Criminal Code regarding sex, her right to choose for her body will trump your right to fuck her. Even for cash. I have already said that just because ads are posted in public does not mean she is available to the public.
Now having said that, and getting back to the point. Is it racist? We actually don't know, so with our laws of innocent until proven guilty, we say no. Is it discrimination? Yes. It that allowed in this industry? Yes.
I openly discriminate against men under the age of 30. I discriminate against women. Almost every escort you see discriminates in some way or another.
I rest my case in this matter. You are free to believe what you want. I am free to disagree.
PS - have a look at some of the Asian places. They are often Asian only, or white only.