William Blake's images are full of vivid color, and expressive lines. Everything a piece of fine art contains. Yet his job title would come more closely resemble that of an illustrator. Which title fits better, or both is really a matter of opinion.
At the time of Willam Blakes life, the paintings that were being produced by the artists of the time were being paid for by the church, or by other means. Yet they never have to be defended if their pieces are fine art or not. The only differences would be the medium, and I don't know any critic that would argue that only one single medium is what can be considered fine art. Even now the art world is slowly having to acknowledge more design oriented works as fine art. That seems to be the factor for most graphic designers.
In todays world, there seems to be an over whelming idea that illustrators are not fine artist. Yet the best designers are generally fine artist before they had the title illustrator applied to them. It really is just a title that more specifically labels what they specialize in. Just like painter uses paint, and illustrator illustrates.
There for it seems the only difference between them is a label, which isn't that important anyway. I would have to say that he is both, since really he fits both categories.