There is a lot to unpack when it comes to this topic and though Reddit allows for in-depth analysis and discussion, it still has a character limit. So I'll try to keep my answer compact and to the point:

Why was the Nazi aryan race (blonde hair and blue eyes) reinforced even though Hitler had black hair and eyes.

It is a common misconception that the Nazis considered blond hair and blue eyes to be racial characteristics of the Aryan race. In reality the Nazis considered facial features (such as an angular jaw, a narrow nose, high cheek bones, etc) and doliocephalic (round) skull shapes to be the key indicators of the Aryan race moreso than hair and eye colour.

That said, the Nazis did depict the ideal Aryan as the Nordic Aryan - this being someone with blond hair and blue eyes as light hair and eye colours were associated with the Nordic race. The Nazis tended to idolise the Nordic ethnicity as the purest blood within the Aryan race (which encompassed not only Germans and Austrians, but also included Danes, Swedes, Norwegians, the Dutch, the Flemish, and the British) and categorised their racial heirarchy in accordance with which group had the most Nordic blood - with Germans being at the top of this heirarchy regardless of the amount of Nordic blood in them. Though Hitler had black hair and blue eyes, this was not something that disqualified him from being an Aryan or even a pure Aryan, it just indicated that he was not a Nordic Aryan.

Is the aryan concept one that was established before Hitler and the Nazi regime?

In essence, yes. The concept of the Aryan race was something that existed before the rise of the National Socialists. The idea of a superior European race is something that dates back to the Renaissance and was significantly developed during the 17th and 18th centuries as a result of the genres of thought that surrounded European colonialism. The idea of an Aryan race, this being a master race of Europeans but not made up of all Europeans first appeared with the advent of Social Darwinism and the first stirrings of the Völkisch movement in the mid to late 19th century. The first mention of the Aryan race (to my knowledge) occured in 1861 with Max Müller's Lectures on the Science of Language - however, it must be said that Müller did not believe in an Aryan race as a superior / master race, but rather saw it more as an anthropological group.

The concept of the Aryan race took on more of a racist / supermacist context in the late 19th century and the early 20th century. Though the concept of the Aryan race was already an established racial concept, the National Socialists significantly developed it into what it has become infamous for; with the concepts of its characteristics, features, traits, and heirarchy being more or less solely a National Socialist invention.

Why was this implemented into Nazi propaganda and law?

National Socialism was a deeply Social Darwinist movement, as are fascist ideologies at large. They saw the competition between nation states as being the same as the Darwinist struggle between organisms for survival (this being the concept of survival of the fittest). As a result they saw the survival (purity) of the race as being paramount in their ideology, it was for this reason that they took great efforts into ensuring that the Aryan race was strengthened through purity laws, the eradication of 'degenerate' influences (such as Marxism and Liberalism), 'proper' education (this being indoctrination into National Socialist ideology), and population growth and colonialism (hence the preoccupation with Lebensraum).

When it comes to propaganda, the image of the ideal blond haired blue eyed Aryan simply served as the poster boy of National Socialism. It was not supposed to be indicative of Aryans as a whole, but rather, was supposed to be the image of what pure Aryans aspired to be.

Hopefully this answer is helpful.

Sources:

Hitler, Adolf. Mein Kampf, edited and translated by Michael Ford. London: Elite Minds Inc. Publishing, 2009.

Kallis, Aristotle. Fascist Ideology: Territory and Expansionism in Italy and Germany, 1922 - 1945. London: Routledge, 2000.

Mineau, André. Operation Barbarossa: Ideology and Ethics Against Human Dignity. Amsterdam: Rodopi, 2004.

Paxton, Robert. The Anatomy of Fascism. New York: Vintage Books, 2005.

Stone, Jon R. The Essential Max Müller: On Language, Mythology, and Religion. New York: Palgrave Macmillan, 2002.

Edit: Spelling