There seems to be - why am I not surprised? - little information and much of it contradictory. While everyone agrees that Heinlein had a rare blood type, few specify what it was. The rarest blood type, at 1% of the Caucasian population, is AB negative.
But that is not universal recipient; it can only receive from Rhesus negative types. AB+ is the universal recipient - for red blood cells. To complicate matters, the situation is , where AB is the universal donor.
A negative type is still a tough hand to be dealt because negative types are rarer than positive types, but several types are harder to donate to than AB-.
Back to Heinlein's type. Some pages say he was AB+ (e.g. https://www.comic-con.org/toucan/10-rob ... ercon-2016); some say AB- (e.g. http://www.fanac.org/Denvention3/blooddrive.html). says it was "A2 negative"; the 2 does not appear to be a factor in compatibility. agrees. I am tempted to go with AB- because that was Eunice's type in IWFNE, but that may have been exercising authorial license; when I was young there was some buzz around having an AB- type.
So what is the worst type to be for donations? Answer: O negative, which can only receive from O negative, which is 7% of the population. But it is the universal donor. So the type that would have made the most sense for Heinlein to be activist about would be O-, but it seems certain that whatever he was, it wasn't that.