Her article in thepeerage is here. She married in March 1789 and passed away later that year.
From the Fitzwilliam Museum; spots removed from background with Photoshop by gogm.
Lady Maria's dress shows a hint of a tent bodice. This and her long coiffure are signs that the British are not conceding to French Revolutionary style. The blue ribbons seem to be a modest revival of Renaissance era rolled or tabbed sleeves.
Keywords: Engelhart, Maria Blunt, Lady Cockerell, Blunt family, Cockerell family, long curly coiffure, head band, scoop décolletage, bertha, epaulettes, rolled sleeves, tight sleeves, tent bodice, necklace