William Blake was definitely a symoblist and a realist. One of his themes is the energy developed from contraries, so it is in keeping with his poetry that he could philosophically be a realist and a symbolist. I'd say Blake was an indirect Realist. He preferred a more clearly defined visual art, which is certainly a more empirical view of art than, say, an impressionistic one. At the same time, Blake believed in the primacy of the inner life of the individual over the socially constructed (and thereby, oppressed) view of the empirical world. I call Blake an indirect Realist because he believed so powerfully in contraries and in the individual perception that he thought the individual's perception itself imagined reality. This is a more metaphysical (while still being physical – contraries) take on ‘the world is what we make it.’ Like symbolist thought, Blake was interested in spiritualism and the perception itself, so Blake has no problem being labeled a Realist as long you label him a Symbolist in that imagination (inner) plays a role in the perception of the world (outer).
In his poem “London,” Blake empirically describes a gloomy scene in London (Realism), but the implication is that this reality is the result of a socially oppressed reality. Therefore, overcoming that oppression requires a vision outside that society to overcome that oppression. Outside that society would be Inner (mental) vision of an individual(s); Symbolism. Blake wanted to symbolically imagine a better outer Reality. Blake frequently used symbolic icons such as angels and devils to foster a more energetic vision and wider perception of reality.