When did "blonde" become offensive?

From this Slate.com(1) article titled "When Did Blondes Get So Dumb?"

Probably since the late 19th century. In 1868, a British burlesque troupe began performing a spoof of the Ixion myth at the Wood's Museum theater in New York City. Featuring four blondes prancing around in tights, Ixion became an absolute sensation and outraged moralists who felt the girls were talentless wretches celebrated only for their bodies. The Ixion actresses were commonly referred to as the "British Blondes," but at some point in the ensuing decade, the term "dizzy blonde" started cropping up: slang for the sort of risqué stage performer that the Brits had helped make popular, and more generally for "professional" beauties, with "dizzy" meaning foolish or stupid. Hence in 1889 the Kansas Times and Star noted, "Many of the local clergy last night warned the church members against a 'Dizzy Blonde' company coming to one of the theaters soon

(1) Slate is a daily magazine on the Web. Founded in 1996, it is a general-interest publication offering analysis and commentary about politics, news, business, technology, and culture.

It's why artificial intelligence is a technique that consists of dyeing blond hair brown.

In general, nouning an adjective runs the risk of sounding offensive. Compare "John is a gay" or "We have to make sure the gays like it too" to "John is gay" or "We have to make sure the gay customers like it too." So saying "She is a blonde" would be a little rude even if there was no insult associated with blonde-ness. But there is (as the other answer details) and so making it a noun is a bad thing.

This is not the same as "you can't say blonde". "I love how that sweater contrasts with your blonde hair" is fine between people who discuss appearance and clothing choices like that. "She looks amazing with blonde hair" is fine in contexts where your opinion on how she looks are ok, which isn't all of them. But calling someone "a blonde" is not that. Further, suggesting that a moment of bad judgement, forgetfulness, or distraction is "a blonde moment" or someone "being blonde" is offensive even when blonde is being an adjective.