There is no contradiction in both believing that humans only form one single race, and also that some humans oppress other humans based on erroneous beliefs about skin color and heritage. The important part is to simultaneously understand that this oppression is occurring while also understanding the the oppression is scientifically incoherent.

Similarly, there is no contradiction in both believing that humans should have universal healthcare without restrictions based on genitals or pre-existing conditions, and also that sex chromosomes are used as a societal excuse for genital mutilation, sexual assault, harrassment and discrimination, etc. Again, the oppression both happens and makes no scientific sense, and there is nothing logically wrong with this stance.

I'm blind to race and sex -- I don't inherently weigh the value of people by their skin color or genitals. I'm not blind to race and sex -- I know that racists and sexists exist, and I antagonize them in order to reduce their effects in society. The conclusion is that the idea of being blind to social constructions is a bit of a rhetorical gimmick!