As an American referee, I'd like to add just a little bit of context.
It is correct that many US referees wear blue wristbands. We wear those in honor of former US FIFA referee
Terry Vaughn, who has battled Huntington's disease. Blue is the color for Huntington's disease, so we often wear blue wristbands with "HD" on them. Occasionally, some referees will wear pink wristbands in October for breast cancer awareness. As for white or black wristbands, I've watched referees around the world wear both colors.
Here is a photo from the article I linked.
View attachment 4628