Oxford’s Ashmolean Museum has always been a place of surprises, despite its severe façade. Perhaps this has to do with its history, the coming together of two seventeenth-century institutions, the University Art Collection, stuffed with portraits of bewigged dons, and the sprawling cabinet of curiosities amassed by Elias Ashmole. The latter was based on the collection gathered by the Tradescants, father and son, famed gardeners and plant collectors, who put it on show at their Lambeth home as “Tradescant’s Ark,” allowing the ribs of a whale to share space with the hand of a mermaid, poisoned arrows, and agate goblets. Since the Ashmolean’s stunning extension was completed in 2009, walking up the curving staircases and circling through the galleries and across glass walkways feels like wandering through the whorls of a shell, mother of pearl, glowing with treasures. All of which has made it an absolutely fitting place for “William Blake: Apprentice and Master,” an exhibition that is at once didactic and very strange.