Maybe he'll get to be on the US Supreme Court. They're always in the market for some backwards-looking bigots.

People’s lines on race - or other forms of bigotry - can be really weird. It’s possible he’s fine with you being biracial but doesn’t care to have a different race’s blood in him. It’s confusing, but then again, so is racism. People need to just let go of racism and find other ways to judge people, like by outward signs of the choices we make. Looking at you, people who sweeten their iced tea.

If I were that nurse/doctor I'd inform him that 1. there's no difference in blood based on the ethnic or racial origin of the donor. 2. That information is generally not available and 3. If he wants to refuse the blood transfusion on such grounds that its his medical right, but that he won't survive that decision, and leave it at that. If he wants to let his racism cost him his own life, that's his choice. Sorry your grandfather is such a terrible person.