i would say that this is best answered by definition of terms and evaluation of subject. either he fits a definition or doesn't.
i don't care for the conventional view of art. i am forever wrested by the term fine Art. this because i have a broad view of the subject. i have been given the understanding that the word fine refers to the literary end, or ultimate purpose, of its creation. that is to say art, the expression of creativity or more simply any creation, becomes fine when it is created for the sole purpose of self expression. if that is the case then does media matter? i firmly say NO. then, that being the case much of the conventional fine art is Not Fine art because it has been done with some other purpose than that of self expression. this means that some of the great so-called fine art pieces, such as the sistine chapel which was a commissioned piece, are not fine by strict definition. by the same definition works created with the intent to be sold have by some degree been tainted from sole self expression. works by students done for a grade then fall outside that strict definition of fine art. that being the case, we have to loosen the definition to account for the fact that we all have to eat, buy supplies, pass classes ect. so then do we say that works commissioned for money, intended to be sold, done by class assignment or otherwise are considered to be fine art if part of the purpose is that of self expression? i say yes. does the medium of a work dictate its validity as art? say No. i say that one is not either a fine artist or an illustrator, but rather can be both. if a man designs a hammer with aestetic quality in mind and it expresses the qualities he desires (self-expression) than, even if the hammer is sold by the millions at $1 a piece i believe his hammer is as much fine art as any.
therefore, william blake was both an illustrator and a fine artist