What makes Marguerite think Blakeney will never escape Chauvelin?

Marguerite sees her husband as an ordinary man, who is concerned with fashion and entertainments. She does not see him as someone capable of defending himself..... someone who intuitively knows when he is in danger.

Sir Percy Blakeney had travelled a great deal abroad, before he brought home his beautiful, young French wife. The fashionable circles of the time were ready to receive them both with open arms; Sir Percy was rich, his wife was accomplished, the Prince of Wales took a very great liking to them both. Within six months they were the acknowledged leaders of fashion and of style. Sir Percy’s coats were the talk of the town, his inanities were quoted, his foolish laugh copied by the gilded youth at Almack’s or the Mall. Everyone knew that he was hopelessly stupid, but then that was scarcely to be wondered at, seeing that all the Blakeneys for generations had been notoriously dull, and that his mother had died an imbecile.