From a thing that showed up in the very late Middle Ages, called the “Letter of Lentulus,” which was presented as being a report from a Roman official (with a known historical name) to Tiberius, about the appearance and habits of Jesus. The letter was taken as eyewitness testimony by many, and the letter in some versions described Jesus’ hair and beard as “fair” and his face as “the color of wheat.” (Although other versions described His hair and beard as “the color of a ripe hazelnut”, ie, those things on the Nutella jar that are light brown and not at all fair. They also described His complexion as “reddish,” which had the symbolic meaning of someone optimistic, energetic — and honest enough to be able to blush.) So it wasn’t racism, so much as popular scholarship and Biblical fanfic (aka “pseudoepigrapha”), that led to blonde Jesuses.
The appearance of blonde or white-haired Jesuses in previous Christian art had always represented the Jesus of the Book of Revelation, Apocalyptic Jesus, Whose hair represents Him as ancient and eternal, or transfigured in light, and Who is dressed for His office as the eternal High Priest. Also, scary and impressive. Either way, His Divinity becoming as visible as His Humanity, rather than how He looked in His life normally on earth.
Most Western art follows the tradition of a bearded, dark Jesus because that’s how the Mandylion of Edessa looked. Ditto the Shroud of Turin and the byssus veil thing. Pictures and relics should generally outweigh literary descriptions; but the Lentulus letter was popular in Germany, a fur piece over the mountains from the Shroud and the byssus veil.