I got a Google hit for something like "spain three kings blackface offensive." Yep, it is true that traditionally in Spain, one of the Three Kings paints his face black.
The story is that every city has a parade on Three Kings Eve, Jan. 5, when the Three Kings (the Wise Men from the Bible), all dressed up in flowing robes, ride through the streets on their way to bring presents to all the good little boys and girls and caca i carbó (poop and coal) to the bad ones. (Now, the poop and coal are meringue candy with brown or black food coloring.) In Spanish tradition, one of the Three Kings is blond and wears a blond wig and beard, and one of them is black. Since black people in Spain were extremely rare until about five years ago, and are still rare in a lot of places, they usually get a white guy to black up in order to be the black King--I think Baltasar is his name. The other two are Melchor and Gaspar. As far as I know, here in Barcelona they get a real black man to play Baltasar, but in smaller places they paint someone's face.
Come on, people, this isn't offensive. They're not making fun of black people here; they just need someone to dress up as a black Wise Man. Remember, the character of Baltasar is a good guy, one of the wise Kings of Orient who brings pressies to the kids. He doesn't do a minstrel-show Stepin Fetchit act, he throws candy to the kids lining the streets and waves at them.