I have a couple of earlier answers that might interest you! First, medieval Europe nurtures a very white Jesus, although the blonde/light hair becomes popular largely in the late 14th century. Second, I've discussed how we got today's stereotypical portrait of Jesus with light skin, that sort of blonde-brown hippie hair, the kindly expression!

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• When did Jesus start being portrayed as white?

During the western Middle Ages in particular, Christian icongraphy was a major vehicle for working out concepts of racial difference before "race."

Medieval European (and Middle Eastern, for that matter) scholars inherited from antiquity the idea that climate affects the development of those native to the region--this will play an important role in the evolution of scientific racism into the early 20th century. The closer to the equator, the hotter the temperature--the angier, earthier, and darker-skinned people got. Centuries before "black" and "white" accrued their modern American meanings, medieval Latin writers were describing and illuminators were painting white and black people. There are a few cases, like the famous Spanish manuscript known as the Book of Games, where the depictions seem to mirror the diversity of the painters' experience to some extent: the illuminations feature dark-skinned and light-skinned Saracen men, and light-skinned Christian and Saracen women alike (the Christian men are all light-skinned). But especially when depicting situations of Christians vs. Saracens, as in illustrations to the Song of Roland, the Saracens are uniformly dark-skinned and the Christians light-skinned. The association of Christians as white and Muslims as black persists in literary descriptions. In medieval romance, characters with one Christian and one Saracen parent are even described as black and white spotted. Or when a Saracen character converts to Christianity, their skin color physically changes from black to white.

So European artists were well aware that native residents of more southern lands tended to have darker skin. And yet western European medieval Jesus is white. It's 14th century mystic Birgitta of Sweden who gives us, through her visions, the tradition of Mary as not just white but blonde. Her Jesus, too, shines with white light.

I want to highlight a second iconographical development over the later Middle Ages. Iconography is all about the use of visual shorthand to convey meaning. In earlier art, into the high Middle Ages, depictions of Jews are recognizable above all by their hats, sometimes also by their clothing. But over the later M.A., Jews start to take on what we now see as "stereotypically" Jewish features (stereotypically thanks to Christian-incubated iconography), primarily darker features and a hooked nose.

Medieval Christians knew Jesus was Jewish. Some theologians and pastors occasionally made a big deal out of it (typically in an anti-Jewish context). Do medieval illuminations and woodcuts of Jesus and Mary give them the stereotypical hooked nose?

No, they bestow that feature on demons and the devil.

Depictions of Jesus are iconographical: they represent concrete theological and often political claims. In western medieval art, the depiction of Jesus as "like us" reigned supreme and served to demarcate, isolate, and condemn everyone else.

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• The Ariel controversy and the backlash got me thinking. Why IS Jesus portrayed as a white hippie?

For the image of Christ behind Space Jesus, we have two 20th century Americans and a whole media empire to credit: Warner Sallman and Richard Hook.

Sallman was a Protestant (Lutheran, IIRC) commercial artist raised in the era (one of the eras) of "muscular Christianity," who spent a lot of his free time in the 1910s-20s making sketches of Jesus. In the mid-20s, he was inspired by a reproduction of a late 19th century Lhermitte painting.

Apparently Sallman saw in the depiction of Jesus, specifically, a more human Christ that people could relate to on a personal/person-to-person level. He produced a "headshot" version titled the Head of Christ.

The image was hugely attractive to a whole lot of people--but it was also enormously publicized, which is why so many people had the opportunity to find it attractive. In particular, several major US publishing houses got behind spreading that particular portrayal to US churches. Sallman produced a whole series of variations that were the 1930s-40s version of the Head of Christ photoshopped into various scenarios. And most in/famously, the Salvation Army distributed the image to soldiers during World War II.

Then the 1960s happened.

The backlash against the Head of Christ was twofold. First, this was definitely an era of Masculinity in Crisis (masculinity is always in crisis). And a lot of people started to grumble that Sallman's Jesus was Girly Jesus. David Morgan, the scholar who very literally Wrote The Book on the modern iconography of Christ (actually, several books), collected a whole series of [very sexist and heterosexist but nevertheless] hilarious insults whipped at the poor image:

• "A pretty picture of a woman with a curling beard, who has just come from the beauty parlor with a Halo shampoo"
• "A bearded woman with as much dignity as a movie-house billboard"

...Please note that at the time Sallman painted the Head of Christ, it was advertised as "the most attractive masculine representation of the Savior."

Again: masculinity is always in crisis.

The second factor of the backlash against Head of Christ, and probably a good chunk of the real reason behind the first, was generational. The 60s/early 70s are the counterculture decade, after all.

So along comes Richard Hook, who's grown up in an era where Head of Christ is all white American Christians' mental image of Jesus, including his. He paints Hippie Jesus (sometimes known as Surf Jesus)--a variant on Sallman's Christ. As Eileen Luhr puts it in Witnessing Suburbia:

In the painting, which differs from other postwar depictions of Jesus in its depiction of a "masculine" Christ, an Anglo-Saxon Jesus looks straight ahead and appears about to speak. He has long, unkempt hair, a beard, and even his clothes, which resemble a hooded sweatshirt made out of rough hewn fabric rather than robes made out of finely spun cloth, create an aura of masculinity. Hook presciently painted a hippie Jesus who fit perfectly within the counterculture.

Luhr goes on to discuss the importance of Hippie Jesus at the time, why this particular model of masculinity instead of, say, Bodybuilder Jesus, Arm Wrestling Jesus, or Fight The Commies Jesus:

The idea of a counterculture Jesus, a rebellious Jesus, a Jesus fighting the system, fit very well with the "we're a persecuted minority" mindset of some conversative Christian movements that were gearing up steam in the 60s-70s.

Today, of course, there are an ever-fractalling multitude of images of Jesus--some iconographical and safe (Space Jesus), others iconoclastic and shattering. But Hook's portrayal and its ancestor in Sallman's Head of Christ have hung in there and continued to influence later portrayals.