I know this might sound stupid, but in the episode The Girls School when Blair says "Blue blood would never mix with yours", what does that mean?

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Jo: Do you know what this means? I never have to hear you breath again!
Blair:And I never have to look at your ugly face again!

Blair:We worked at Edna's Edibles, the finest store in all the land.
Jo:Blair, you're talking like a leprechaun.

Jo:Ferra, you're melting my toothbrush!

She was just being mean.

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Can't we all just get along?

Live long and prosper.

What if I die an old maid in Avonlea without ever being kissed...or swooned?~Felicity King

Yeah. That was just Blair being a snob. Bassically it was like a fancy way of calling Sue Ann a farm girl. A blue-blooded person is usually someone who has an aristocratic descent or birth and generally they marry other blue-blooded people.