Jane I have just trawled the net to try to find any references to what I once heard. so far have only come up with two - this one where a poster says that "the references to 'dark Satanic mills' , actually refer to standing stones (Mills as in 'Millstones'). . .. this drawing next to another reference to 'Satan's Mills' make this particularly clear; . . ." (The drawing in this link to an image of one of Blake's poems is interesting)
and this one which has a bit on "Blake's Jerusalem: what does it all mean?" and in it he says he "vaguely remembers from studying Blake's work thirty years ago that Blake not only believed that Christ did visit England, with Joseph of Arimathea, he built a complex personal mythology around this fact and passionately believed that Albion (his name for England) was the second Promised Land, so the first verse means it scarcely seems true, but if Christ did visit England, then there is all the more reason why this country has a special claim to be the site of the New Jerusalem. As for the dark satanic mills, Blake makes it clear elsewhere in his writing that he was not thinking of textile mills in Lancashire but rather unthinking religious institutions, which Blake saw as encouraging the mindless mass production of meaningless prayers."