The novel Blue at the Mizzen is the twentieth and last completed historical novel in the Aubrey-Maturin series by Patrick O'Brian, first published in 1999. It is set after the Napoleonic wars, in the fight for Chilean independence from Spain.

Jack Aubrey and Stephen Maturin, having heard the details of Napoleon's defeat at Waterloo and having collected their share of the prize from their last capture, set sail for the dual mission of charting the Chilean coast and aiding those who seek independence from Spain. Maturin and his colleague Dr Amos Jacob speed the plot as covert intelligence agents and Aubrey's political advisors, while Aubrey makes bold moves in dealing with the factions in Chile.

Reviewers were positive about this novel, finding it "a shining jewel", "an intricate, multifaceted work -- one of those rare novels that actually bear up under close scrutiny." The "deeper sense of the culture of the age" that marks the series is also true of this novel, which shows the "period of deflation, both economic and emotional" for the Royal Navy and its associated businesses after the victory. The new midshipman introduced in this novel, and Maturin's new love, are well-liked. The young man is "a dashing young foil for the ship's elders", who shows that the main characters have aged and their pace has changed. The voyage to Chile treats readers once again "to O'Brian's talent for life on board, incidents and accidents". Aubrey has earned a "just reward" at the end, made an admiral of the blue. Feelings about the series as a whole are also positive, and strongly stated: ""There is nothing in this century that rivals Patrick O'Brian's achievement in his chosen genre."; and readers can so easily enter this fictional world, because "O'Brian did the hard learning long ago and then began to write with fully justified assurance and pleasure."