Blake was considered to be a change of pace for literature at the time. He started writing in a more romantic and Elizabethan way which was a callback to times gone by as well as to what would come post-Blake.
Blake is considered to be a seminal figure of the Romantic movement which departed from writing for form and took a turn towards writing for content. This included writing that was more attached to nature and the world in which the readers were living.
To sum up, the reason Blake is transitional, is because he is considered to be the first writer to start writing in this fashion and change the writing style at the time.