William Blake in my opinion is both fine artist and illustrator. The many works of art that he created for his poems, manuscripts, and even for the Book of Job demonstrate how versatile of a artist he was.
Blake could be considered an illustrator because of the art that he has done for his poems and for the book of Job. The art pieces themselves are not meant to be art by themselves. They are meant to be put with the text that they were created for. Without this text the art almost looses its emphasis. The pieces also have a less serious context to them, like they are meant for the text and not just stand alone art.
On the other hand Blake could be considered a fine artist. He was sort of a jack of all trades when it came to the arts. Whether it was painting, engraving, poetry, and manuscripts. The combination of these could make him a fine artist. As said before he would combine art with his poems. This combination which brings two different medias together in harmony could be considered fine art. Separately the works are in two different categories, but once brought together they form a new category that falls under fine art.
With all this in mind I believe that William Blake is undoubtedly not only a fine artist but an illustrator as well. The works of William Blake push the limits of both these categories of art, yet bring them together in a subtle balance that is hard to dispute.