Somewhere deeply embedded within the manuscript collections of a Florentine library may lie a chapter Niccolo Machiavelli forgot to include in The Prince, headed ‘On the Utility of Bastards’. Base-born children were an asset to Renaissance rulers, whether as a proof of potency, a status indicator or a bargaining counter in the form of spare princesses to be shuffled into marriage for the sake of clinching a peace treaty. The wrong side of the blanket had its distinct advantages and the gods stood up for bastards as cheerfully as Shakespeare’s Edmund in King Lear could have wished.

One of these illustrious by-blows, the subject of this book, was the son of Machiavelli’s dedicatee, Lorenzo di Piero de’ Medici, Duke of Urbino and de facto lord of Florence. Lorenzo, alas, had the bad taste to reject The Prince in favour of a brace of greyhounds offered to him at the same time and disappointed its writer still further by dying of the pox aged only twenty-six. He left a legitimate daughter, Catherine, who as queen of France displayed both Medician savoir-faire and Machiavellian manipulativeness to an almost exaggerated degree. Control of Florence, however, passed to Lorenzo’s bastard son, Alessandro, whose mother, Simunetta, either a slave in the household of a relative or else a peasant girl from a Tuscan village, was indisputably lowborn.

Quite possibly she was also black. Catherine Fletcher’s study of Alessandro’s brief reign (1531–7) charts the zigzags of his reputation in the hands of Florentine propagandists and chroniclers, while also examining the ways in which history has handled the topic of his mixed racial inheritance. The issue of the duke’s colour seems not to have mattered much to his contemporaries. Only in the late 19th century, as theories of ethnicity hardened into a pseudoscience, was the idea of Alessandro as a genuine ‘black prince of Florence’ invoked to explain his more flagrant inadequacies as a ruler. One Italian medical expert, pontificating on ‘the hereditary transmission of biological characteristics’, spoke of ‘the psychic lacuna that made Alessandro a born criminal’. An English historian, meanwhile, wrote him off as ‘incapable, vicious and universally detested’. Each of these harsh verdicts took it as read that a black Medici must be damned from the outset by his skin colour.

As The Black Prince of Florence reveals, Alessandro was less of a dusky degenerate than a victim of the age in which he was born. The Medici heir, living when he did, had little chance of becoming the perfect sovereign, even in the hard-nosed incarnation outlined for his father by Machiavelli. The Italian Renaissance world brought to life in Fletcher’s biography is more redolent of The Duchess of Malfi or ’Tis Pity She’s a Whore than of Leonardo, Michelangelo or Botticelli. Around the growing boy in Rome, Florence and Bologna ranged a high-status rogues’ gallery of popes, princes and cardinals without an ounce of scruple, humanity or altruism between them. Strutting, venal, sottish and psychotically violent, they make us appreciate the need for that moral counterweight eventually posed by the Reformation and the Council of Trent.

Saturnine president over this ghoulish assembly was Emperor Charles V, Europe’s most active potentate and the political arbiter of Alessandro’s destiny. Even before becoming duke of Florence, the black prince had been menaced by the ambitions of a Medici cousin, handsome, smooth-tongued Cardinal Ippolito, whose manoeuvres were reinforced by malcontent Florentine exiles. Once Alessandro assumed control in Tuscany, however, Charles, seeing the need for Italian stability, guaranteed his support with the offer of a duchess in the shape of an imperial bastard, his twelve-year-old daughter Margaret. The wedding celebrations, which the painter and biographer Giorgio Vasari called ‘the sort of show that the court of Heaven might have put on for a long-awaited soul’, were clouded by an inopportune eclipse of the sun, an omen noted when Margaret later suffered a miscarriage. She seems, all the same, to have loved Alessandro in spite of his notoriety as a coureur de femmes.

A lustful escapade brought about his death on Epiphany eve in 1537. By then the scheming Ippolito had been conveniently poisoned on the orders of Pope Paul III, but another cousin, Lorenzino de’ Medici, luring Alessandro to an assignation with the wife of an absent nobleman, hired an assassin, the wonderfully named Scoronconcolo, to finish off Florence’s duke. Alessandro, to his credit, went down fighting, fixing his teeth into Lorenzino’s thumb as Scoronconcolo drew the fatal dagger. Fletcher sees this as an ultimate gesture of contempt; it struck me as the same kind of thumb-biting insult that sparks off the street brawl between Capulets and Montagus in Romeo and Juliet.

Lorenzino’s motives puzzled his contemporaries and remain obscure to this day. Florentine republicans claimed him as a hero; Vasari read the whole episode as an eruption of long-simmering hatred and envy; others sought an explanation in prophecy and astrology. Benvenuto Cellini, never one to miss the chance to embellish a story, claimed to have seen, while out duck-shooting near Florence on the day of the murder, ‘a huge beam of fire which sparkled and gave out extraordinary lustre’ as it hovered sinisterly over the city. Lorenzino himself sought refuge in Istanbul, where he issued an apologia claiming his act had visited justice on a tyrant who ‘called himself a Medici’ but who ‘was not a jot inferior to Caligula in his scorn, mockery and torment of the people’.

Alessandro’s body, dumped in his father’s sarcophagus at Michelangelo’s new sacristy in the church of San Lorenzo, was abandoned to the obloquy of historians. He was in fact no worse a ruler of Florence than most of his Medici successors and Machiavelli would surely have approved of the tough love with which he governed his Tuscan domains. In her search for him Catherine Fletcher is entirely at ease amid the Renaissance world and its archival resources, and her details, particularly those involving dress, feasting and ceremonial, are generously deployed in the work of recovering a neglected episode of Florentine history. Alessandro himself, however, remains obstinately unapproachable. He appears to have left scarcely anything in the way of personal papers, while most contemporary comments on him were glancing allusions rather than the fruits of close scrutiny or recollection. The darkness of the black prince of Florence has less to do with his possible African heritage than with the shadows, impenetrable for now, of historical documentation.