It's like arguing that it's wrong to ban drinking for 18 years old people because there are mature 18 years old drinkers. Surely, there are but the reality is that 18 years old people are likely to abuse alcohol and that's why we increased the drinking age.
Apples to oranges.
Age isn't like race and sexual orientation in that everyone who is older than 18 has once been underage. Unless you die before 18, each person is underage and then becomes of age. They can't drink and then they can. People are black or gay for their whole lives. If they are banned from donating blood, there isn't a day where they can start donating.
That said, it still makes more sense to focus on the accuracy of HIV testing, because even if blacks or gays do statistically have a higher rate of being HIV+, that speaks nothing to an individual case. A donor whose HIV+ blood gets past the screening gets past the screening. You think the person who gets HIV from the donor is going to care if it was a black gay man or a white straight man? No, in that case all that's relevant is HIV+ or HIV-. Statistics can't be applied to every case because they're only as good as probability. Focus on the testing, not the demographics.