Broadly speaking, Black Spring is written in a stream-of-consciousness style, with narration jumping around from topic to topic, digressions into things that are not directly related to the subject or plot at hand, and a specific informality that sounds more like conversation than written prose. This is a specific type of style, meant to evoke powerful imagery and immediacy. An example of this style can be seen early:
In my dreams I come back to the Fourteenth Ward as a paranoiac returns to his obsessions. When I think of those steel-gray battleships in the Navy Yard I see them lying there in some astrologic dimension in which I am the gunsmith, the chemist, the dealer in high explosives, the undertaker, the coroner, the cuckold, the sadist...
(Miller, Black Spring, Google Books)
Note the powerful descriptive words and the digression from discussing "the streets" to remembering time in the Navy. The narrator's memories are tangled up with each other, and one memory naturally (in the mind) leads into the next, but the connection is not as immediately obvious to the reader, who must interpret from the surrounding context. This allows the use of both metaphorical prose, imagery-based feelings (not necessarily "real") and a feeling of urgency that draws the eye from line to line. This style became popular in the 1930s, and Henry Miller is one of its best-known originators.