It is unfair to locate Blake on a political spectrum because by strict definition his theory has nothing to do with politics, just like Thomas Paine’s theory has nothing to do with religion.
In A Song of Liberty from The Marriage of Heaven and Hell, Blake disdains any possible kind of system: empire, any kind of government either democratic or not, church, slavery, monarchy. So I think it might be safe at least to call Blake an anarchist. He does not support any type of institution because institutions set standards.
However, he is definitely not just an anarchist because there is something dominant in Blake’s theory: the Poetic Genius. For here we need to examine the position of religion in Blake’s theory. Blake is against centralized church and religious morality. But he is still a Christian and believes Jesus is an artist and as rebellious as him, as a man who break the ten commands. In the relationship between state and church, he deletes the existence of state and decentralizes church into personal practice. Nevertheless, the religion exists and exists as the ultimate goal of his theory: the New Jerusalem. So Blake is a religious anarchist.
If we characterize all the political theories during that time period as rational, then Blake is a romanticist. The practice of art and imagination, the essence of Poetic Genius are irrational. Blake’s theory of revolution is irrational, thus system does not exist. He calls for the Poetic Genius in everyman and the undisciplined environment. A categorization for Blake is shameful.