by that I mean various methods of talking to or examining people and determining how much pain they're in. in principle there is no reason pain scales ought not to work for women and black people. but in fact they don't, because the tests don't exist in a vacuum. health care practitioners interpret them in a staggeringly prejudiced way, no matter how sophisticated and evidence based the tests are

EDIT: To give a more specific description - there was and is a widespread belief among doctors that black people feel less pain than white people because they have "less sensitive nerve endings." this isn't evidence based. black people get less pain treatment in comparable situations than white people do.

I don't know if there is any myth as particular as the one above about women, but women also get less pain treatment than men. iud insertion and endometriosis especially can be brutal experiences for women because doctors don't take the associated pain seriously.