Blunt identified with the Italian architect Francesco Borromini (1599-1667) as he had with Poussin, who was five years older. Just as Poussin was his "first love," so he confessed (in a doggy metaphor), "Once Borromini has bitten you he never lets go." Just as the French painter had been criticised for excessive reliance on ancient art, so "Borromini was vilified as the great anarchist of architecture, the man who overthrew all the laws of the ancients and replaced them with disorder, and who corrupted the taste of many architects." Blunt used similar methods to rescue both Poussin and Borromini. After discussing Borromini's sources and theories, he gave "a clear account of the artist's career, a convincing analysis of his style and an estimate of his achievement". Blunt based his account of the architect's style on the influence of the Ancients, Michelangelo and (rather vaguely) nature. He argued that the architect struggled "between imaginative energy and intellectual control", and that his bizarre inventions — curvilinear, ornamental and fantastic — "were in fact variations based on an almost [his favourite adverb] ruthlessly logical method".

Borromini was a hands-on stonemason and draughtsman as well as architect. A contemporary praised his diligence, expertise and mental power: "He guided the builder's shovel, the plasterer's darby [leveller], the carpenter's saw, the stonemason's chisel, the brick-layer's trowel and the ironworker's file, with the result that the quality of his work is high but not the cost, as his detractors claim, and all this springs from his intelligence and his industry." But he had a bitter, jealous rivalry with the more charming and successful Gian Lorenzo Bernini, who won all the best commissions. Unable to finish many of his ambitious projects and give complete expression to his ideas, suffering from hypochondria and morbid introspection, Borromini finally became misanthropic, manic and frenzied.

Blunt (with yet another "almost") observed that Borromini alienated his patrons: "Though physically of a fine presence, [he] lacked all the social graces. He was melancholy, nervous and uncompromising, and these qualities soon turned to a neurotic fear of all human contacts and a suspicion of people, which almost reached the stage of persecution mania." Yet Borromini had two things in common with Blunt: his "devotion to art and unhappiness in his life". When describing Borromini's character and career Blunt seemed to be writing about himself. Like Blunt — who desperately tried to cover his tracks and avoid his inexorable fate, and was finally repudiated by former companions who did not want to be tarred by his treachery — Borromini "was a neurotic and unhappy man, constantly dogged by disaster, often largely of his own creating, quarreling even with his best patrons and closest friends".

Borromini was the first major artist to kill himself. After his botched suicide attempt, and with amazing objectivity, he managed to dictate a morbid account of his experience before dying a few hours later. He seized a convenient weapon and followed the Roman tradition — described in Plutarch's Life of Brutus — of falling on his sword: "In despair I took the sword and pulling it out of the scabbard leant the hilt on the bed and put the point to my side and then fell on it with such force that it ran into my body, from one side to the other, and in falling on the sword I fell on to the floor with the sword run through my body and because of my wound I began to scream . . . [Friends] pulled the sword out of my side and put me on my bed; and this is how I came to be wounded."