This is a very rough summary from back in school where we took they apart rigorously. Mistakes likely :)

In short: the blonde/blue eyes is the simplification that was applied quite early. The supreme race were the European Nordic type and that was just one of the most prominent characteristics. That said: it’s VERY pseudoscience, mixing genetics with religion and esoteric believes.

And that “theory” didn’t end with Scandinavian good, rest bad: it was a whole bullshit of how world races work. The Japanese (if I recall correctly) were by the way another super superior awesome “race” in Hitlers book. I think there even was a third one!

Oh and to the personal question: this is like asking someone who was bad why they’re still catholic! I recall that there were several time pieces describing Hitlers sadness and even depressive episodes because he wasn’t part of that superior race (that poor, poor dipshit).

But enter his loyal staff who got phrenologists on board and historians and hurray, the Fuhrer is a direct Arian descendant with all positive traits who just deviated in an irrelevant hair- and eye color pattern. (And size and saneness but for some reason they left that out).

It’s historically fascinating to think about the fact that Hitlers believes and Nancy Reagans “I call the astrologist for every major decision of the cold war” are quite similar in their origins in that regard - with hitler being a tiny bit more extreme of course.