Izabela Lubomirska's Wikipedia article is here. She was born into the Czartoryski family and married Stanisław Lubomirski. Google's translation of Marquise Lubomirska's Polish Wikipedia article states she wore blue skirts, as shown, so she was called the "Blue Marquise."
This is Isabella Lubomirska, née Czartoryska, not Izabela Czartoryska, née Fleming.

1767 Izabela z Czartoryskich Lubomirska by Aleksander Roślin (Muzeum Pałac w Wilanowie – Warsawa, Poland)

Izabela Lubomirska, the Blue Marquise by Per Krafft the Elder (Pałac Łazienkowski - Warszawa, Poland)

ca. 1767 Duchess Izabela Lubomirska by ? (probably Muzeum Książąt Czartoryskich w Krakowie - Kraków, Małopolska, Poland)

"1757" Izabela Lubomirska, the Blue Marquise by Marcello Bacciarelli (Museum Palace at Wilanow - Warszawa, Poland)

1799 Princess Isabella Lubomirska, nee Czartoryska by Slendzinski after Élisabeth Louise Vigée-Lebrun original sold by Christie's (Muzeum-Zamku w Łańcucie - Łańcut, Podkarpackie Voivodeship, Poland)
