Don't we do this with all white singers?

SEATTLE, Washington - I thought that the process of making a singer sound much better on the studio album than live was some dark art of manual manipulation of sound waves. Apparently it's a bit more old-fashioned, at least according to Henry Rollins, who said in an interview with Sun Media that Britney Spears sounds good on albums because they mix her singing with that of a "black chick":

"They have the black chick come in and sing, and Britney sings over it, and they mix them together," said Rollins, who has his own talk show on IFC.
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"Britney apparently isn't actually the worst singer, she just has no feel. So they bring in this older black woman who sings the song, then Britney sings to it, and they kind of make a mix of the two voices, and that's what you hear on the records."

found via F-listed