Hitler discussed this in Mein Kampf.

He promoted the idea of five sub-races of the Aryan people. Dinaric, Alpine, mediterranean, Nordic, and Eastern Baltic. The best kind of Aryan is, you probably guessed it, are the predominantly blue eyed and fair headed Nordics.

This was actually very popular, Nordicism I mean, way before Hitler promoted it. Anyway, Hitler said that Germans and other kinds of Aryans have either been diluted and weakened over time (mixing with Jewish people for example).

He promoted blue eyed and blonde germans as models because they are less not Nordic. Essentially, Germans needed to expunge non-Nordic lineage which will cause them to have blue eyes and blonde hair again.

The issue with Nordicism conflicted with Italy which promoted Mediterreneanism.

see this similar post from a few months ago for previous responses

How is it the Nazis justified the attempted creation of a society filled with tall, blond-haired, blue eyed people, when Hitler himself did not fit this model?

But I thought a strong chin, medium-tall height, and blue eyes were stereotypical Nordic features?