Never heard that black cats are more or less allergy causing, lol.

Black cats -- and black dogs -- are much less likely to be adopted. It is very difficult to read their facial expressions is one reason; another, as already mentioned, is the idea here in the USA that they are bad luck. Funny thing, in UK, they are considered GOOD luck! Go figure, lol.

Many shelters will not adopt out a black cat at all during the month of October. Sadly, black cats are at high risk around Halloween, so to keep them safe, the shelters hold on to them.

If you want say, a Devon Rex, or another low allergen type, why not check if any are in a rescue? Use Google and put in
devon rex rescue
or which ever other breed you are thinking about.