The long tradition in English literature of compromising African humanity in order to examine the European psyche is cumbersome baggage for a writer tackling the transatlantic slave trade. Too cumbersome, in fact, for Bernardine Evaristo, who has dumped it. And, in an act of supreme imaginative daring, she has cast Europeans as the slaves in this alternate history.

Audacious genre-bending, in-yer-face wit and masterly retellings of underwritten corners of history are the hallmarks of Evaristo's work. They are in abundance in her debut novel, the verse narrative Lara, and in The Emperor's Babe, which follows the story of Zuleika, a Sudanese "It Girl of ancient Cheapside". All of which makes the leaden prose and gossamer-thin characterisation of the first two-thirds of Blonde Roots so disappointing.

The plot centres around a white English slave, Omorenomwara (née Doris Scagglethorpe). Her life is bisected by an attempted escape and traumatic recapture by her sadistic black Aphrikan owner, Chief Kaga Konata Katamba I, or Bwana, to his employees. In book two, we read Bwana's story of his own life and slave-buying missions to the dark heart of Europe - a parody of Heart of Darkness with Bwana in the role of Marlow. Their lives span three points of a triangular slave trade: the Cabbage Coast in England; Londolo - the capital of the United Kingdom of Great Ambossa off the Aphrikan coast; and the West Japanese Islands near Amarika.

The fact of Doris's Englishness, or, as it registers here, whiteness, is fleetingly engaging but quickly begins to feel gimmicky. Our attention is so firmly drawn to the physicality of characters, rather than their inner lives, that we gawp rather than empathise. The inversion of African and European roles is literally skin-deep: big questions about economics and the cultural impact of the real-life slave trade are dodged entirely as continents are simply reversed wholesale. The question Blonde Roots poses is not so much "What if Africans had enslaved Europeans?" as "What if Europeans had Black skin?". To which the answer appears to be: "Not much."

As a "house whigger" in Londolo, Doris becomes a walking mouthpiece for heavy-handed socio-anthropological observations such as: "In the Burbs you rarely saw a free whyte with natural hair. They wore the perms, twists and braids of Ambossan women, although afros were most in demand. The hairdressers used kinky Aphri kan hair on the Burbite women, who had their own fine hair chopped off . . . when the blonde, red, brown or straight roots came through it looked just plain tacky."

Lacking the vivid historical detail that brought zing to Zuleika's Londinium, Doris's Londolo feels about as vibrant and real as Milton Keynes. Evaristo's amalgam of historical London and modern-day Mombasa results in little more than a quirky Tube map (Mayfah, Edgwa District, paddinto, 'M'Aiduru Valley). We are never allowed off the tourist trail into the backstreets and inner lives so richly imagined in Incomparable World, S I Martin's evocation of historic black London. Only once, as Doris escapes through the menacing cityscape of her oppressors on foot, could I envision her as Mary Prince (the abolitionist who in 1831 published an account of her life as a Caribbean slave) on her desperate walk across London from her owners' house to the Moravian church in Hatton Garden.

The writing, characters and dialogue finally come alive in book three. It opens with a cinematic wide shot of the River Temz, teeming with crocodiles, hippos and all manner of human life:

skateboarding teenage boys with wild aphros, beaded corsets and leather jockstraps hurtled up its sweeping walls, turned, hovered mid-air, then skidded down again with a whoop and flourish. Crouched in the shallows was the after-dinner crowd, chatting and shitting. The girls wore their hair shaved with a bushy topknot just above the forehead, their faces painted a bright yellow with white chalk dots circling each cheek and kohl-black lipstick.

Here, almost buried at the end of this baggy book, is a rather good novella about the lives of a tight-knit slave community on a colonial plantation. Evaristo's keen ear for the musicality and rhythm of the spoken word finally comes into play with Ye Meme, a "glamazon" who takes Doris under her wing, introducing her to her cronies thus: "Ladyies, dis here mi new skinnie-like-bamboo frend, Omorenomwara. Lissan, all she do iz work, eet an sleep. She na talk, she na laf, she don't let none a-deze wotless brudders about here poke her in de bushes. Mi gyal here iz sofistikyated, ya hear?" The conventional story-telling is a welcome relief after so much high-concept experimentation, but, for this reader, it's too little, too late.