JESUS, BEFORE DEATH:
The bible's only description of Jesus, prior to death, was that he was a plain looking man.
Genetically, Jesus was purported to be the son of God, and Mary (married to Joseph at the time....remember, "thou shalt not covet they neighbor's wife)." Thus, Jesus was half Jewish and half God. Unless, of course, God wanted to make a miracle and make Jesus any race he chose. Assuming that Jesus was half Jewish (Semitic), he would have looked like a Semite (which, back then, was like a modern Arab (especially Bedouin, since they have mixed less than other Arabs).
Since the diaspora (flight from Egypt), Jews were strewn to many parts of the world, including Russia and Germany. There, they interbred with natives (such as Germans), and took on the look (and genetics) of Germans. Thus, there are blond Jews. It is difficult to decide who is allowed to be an Israeli citizen based on DNA that has been so diluted, but Israelis shun Hitler's ideas of racial purity.
CHRIST AFTER DEATH:
Am I talking about Jesus after death? No, Jesus was in the ground, dead, and remained dead. Jesus was the human vessel of the spirit of Christ. But, after crucifixion, Christ (a spirit without a body) rose from the dead and appeared with bright blond (or white) hair, red eyes, and bronze feet (ghost with a tan....cowabunga dude).
HOW TO PORTRAY JESUS:
Paintings have to make him look like something (or maybe make him like Wilson (Earl Hindman), on the TV show Home Improvement, who almost never appears without his face hidden). Bigotry likely played a role in making Jesus look more white and less Semitic.