Claims that there was a 40% rh- frequency among the Yamnaya has only been a guess by a scientist often quoted. It did however alert enough people to assume that this is a fact. It could be, but again: It doesn’t help too much to present something without proof leaving us to guess or argue its validity.
The Proto-Basques didn’t have Yamnaya DNA. The Yamnaya later invaded the Basques making them what is now one of the populations highest in the Yamnaya R1b y-DNA frequencies. So the question is as follows:
What were the blood type frequencies among the Proto-Basques before the Proto-Celtic invasion?