Quoting myself from previous instances of this question:

Blue eyes and blonde hair was an ideal, but German propaganda actually tended to downplay physical features when discussing the ideal Aryan. A Hitler Youth booklet called "Faith and Action" says: "Race means to be able to think in a certain way. He who has courage, loyalty and honor, the mark of the German, has the race that should rule in Germany, even if he does not have the physical characteristics of the “Nordic” race. The unity of the noble soul and a noble body is the goal to which we strive. But we despise those whose noble body carries an ignoble soul."

Hitler's actual definition of 'Aryan' sometimes changed based on what was politically convenient. Originally it was "white, non-Jewish" but it became necessary for him to add Slavs to those who were white but not Aryan. The lack of a specific definition allowed him to change it when it was convenient, as was the case in Yugoslavia with the Serbs being considered the "most Aryan" in the region... until a Serbian anti-Nazi coup forced him to declare the Croats the most Aryan.

/u/Gadarn gave a pretty full answer but I'll tack this on: the Nazi conception of race was very different from modern, especially North American standards. Nazi racism was paradoxical because it was based in racial pseudo-science as much as it was based in spiritual mysticism; they (and many other Germans) believed that races had inherent moral traits. Those descended from the Aryans had the most noble and desirable traits, while the Jews were considered to be fundamentally morally flawed because of their race.

It didn't matter to the Nazis if a Jew had blonde hair, blue eyes and stereotypical 'Aryan' facial features and a Hitler-mustache - just being a Jew or having Jewish blood automatically made them a threat to the Volksgemeinschaft ('people's community', a propaganda term for Nazi totalitarian-racial society). To the Nazis, race came with inherent moral and psychological traits that were impossible to fully overcome - a German would always be inherently noble, courageous and devoted, while a Jew would always be inherently cowardly (but somehow always conspiring to destroy the German race) and evil. Those with Aryan blood were culture-creators, while Jews were culture destroyers (nevermind all those Nobel Prizes).

It should also be clear that the Nazis' did not have a monolithic racial doctrine. Some were more into the pseudo-science aspect, while others like Alfred Rosenberg and Himmler were exponents of a more spiritualistic and mystical racism. Rosenberg was considered the architect of Nazism's general racial doctrine, and wrote about how the Aryans were an advanced race from Atlantis who were responsible for all of humanity's achievements in China, India, Greece and Rome until they mixed their blood with the natives and degenerated. Nazism was weirdly multi-national in its racial ideology - Frenchmen, Italians, Russians and even the Chinese had a small percentage of the population with Aryan blood, which is why they were totally OK with creating multi-national SS divisions. He believed that Germans still had more Aryan blood (note that Germans and Aryans were not the same thing; they believed there were Germans without Aryan blood, and that other nationalities had it. Germans simply had a higher percentage of 'Aryan-ness' than everyone else) and that they had to guard their precious blood from contamination with inherently evil Jewish blood. This kind of racial worldview had a very strong spiritual dimension that even other Nazis were wary of.

All of this meant that a German with brown eyes and black hair could still have a high percentage of Aryan blood, making him inherently good. A Jew could also have Aryan blood and by extension, Aryan features, but his Jewish blood would always overpower his Aryan blood, which is why the Nazis passed the Nuremburg laws prohibiting inter-racial relationships.