It could be argued that William Blake was an illustrator. After all, he did illustrate the Book of Job in the Old Testament of the Bible, which is one of the main things he is credited for and he illustrated his own poetry. The fact that he was an illustrator is pretty hard to argue with.
However, William Blake was a poet, engraver, painter, AND a manuscript illustrator. Perhaps the most impressive aspect of his art though wasn't all of the poems, painting, engraving, and illustrating he did, but the combining of all of these things together to create one whole work. He was extremely well rounded to say the least.
After viewing some of his work, I felt that giving him the title of an illustrator was almost insulting him. His work goes so much farther than illustration and so much went into every piece. The detail in his works are unbelievable and could rival those of the great fine artists in history.
He was a creative, well-rounded, and needless to say talented individual and in my opinion, a great fine artist.