Visitors to the new William Blake exhibition at Tate Britain – a show that we are contractually obliged to refer to as “blockbusting” – are warned beforehand “Please be aware that the art of William Blake contains strong and sometimes challenging imagery, including some depictions of cruelty, suffering, sexual violence and the brutal treatment of enslaved people”. Well, this is all rather more dramatic than, say, a Constable exhibition. Blake occupies an exalted place in English popular imagination, and rightly so. As both a peerlessly talented artist – that overused word “visionary” is for once entirely accurate – and a poet whose writings alternately stir and chill, he has remained a subject of fascination for other writers, artists and musicians ever since his death in 1827. This retrospective, the first major one in three decades, attempts to explore why we are all, in our varying ways, in the shadow of Blake.