When did "Black" start replacing "negro"?

[Sam Clements]

We're in a debate over at the Straight Dope.

A poster pulled up what appears to be a cite from the Random House Unabridged Dictionary:

" In the late 1950s BLACK began to replace NEGRO and today is the most widely used term."

What percentage of polite people would have used "black" instead of "negro" in 1959?  I didn't think "black" was used in any significant way until the mid-late 1960's.  But I'm willing to listen to any evidence.  

I guess what I'm looking for is just "how significantly" did the word "black" enter into the language before say, 1965?

Sam Clements

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