It's a literal meaning referring to the idea of a secret ruling class of leftist women in society. They're assumed to have all this societal influence and control ("blue-haired gestapo") despite no actual leaders resembling this. I feel like the burden of proof is pretty high for claims like this.

Maybe it's just another way for them to revolt against society, or they just really like the way it looks. I don't really know why but dying their hair in some outlandish color is pretty popular with that crowd. I'm not against it by the way, it looks nice when done well.

Amusingly enough, “blue hairs” has long been slang for elderly women - a lot of old women used to put a bit of bluing in their yellowish grey hair to make it look whiter/more silvery, and some of them overdid it a bit at times.

What’s bluing? A non-permanent blue dye that used to be used when rinsing white shirts to make them look less yellowed. It’s still sold, but I don’t know anyone under 60 who still buys it.

Amused by this shift in meaning, mostly because I’m now the female equivalent of a “grey beard” in tech, old enough to have experienced the Dotcom bust - there is a fair bit of silver in my hair, and I can’t be bothered to cover it. “Grey bun”?

People that care about these topics are often slandered as "women with dyed hair." It is a way of creating an outgroup.