In summary, the legend is this: Joseph of Arimatheia was a rich man, a relative of Jesus (and one of his covert disciples), who after the Crucifixion claimed the body of Jesus from Pilate. He came to Britain with other disciples and founded the first British church at Glastonbury, where he planted his staff. This miraculously flowered into a tree, The Glastonbury Thorn, whose offshoots may still be seen today, flowering every Christmas. (A sprig or cutting is sent to Buckingham Palace every year from this tree, which analysis has shown is a Palestinian variety.) Joseph also brought and kept there certain sacred relics, perhaps the Chalice Cup or Grail. He knew Britain from his trips as a tin merchant, and in fact, on one of his trips he had brought his nephew, the boy Jesus. Joseph, and some say the Virgin Mary, is said to be buried there, along with the Grail featured in legends of Arthur – whose official tomb is still to be seen there.
Although Blake's own source of inspiration may lie in the rumour (perhaps begun by him) he was a Druid, the 'Holy legend' first surfaced in print in the Grail romances of the early Middle Ages. There was a Romance from around 1200 called Joseph Of Arimatheia, depicting him and his followers (not the Church) as Keepers of the Grail, never reaching Britain but founder of a secret Order whose members in the "vale of Avaron" knew the "secret" of the Grail -- the words which will end the "enchantment of Britain." The High History Of The Holy Grail, alias Perlesvaus, of c1225 AD, and later Romances, even imply a dynasty from Joseph and Christ to Sir Galahad.