"Let's see. Purple. For your hair. Can't tell this is a goddamn anime."
—Da Vinci, Ghost Stories
Wouldn't you wonder if your "great sword teacher" looked like some psycho-cyberpunk with green hair?
— Kendo Rage manual
Once there was a boy who woke up with blue hair
To him, it was a joy—until he ran out into the warm air
He thought of how his friends would come to see
Would they laugh—or, had he got some strange disease?
— Crash Test Dummies, "God Shuffled His Feet"
So what is this? Super Saiyan with blue hair dye?
— Freeza, Dragon Ball Z: Resurrection ‘F’
His hair color is merely manga artistic expression. He is fully Japanese.