In chapter 1 of The Scarlet Letter, the black flower which is mentioned is allegorical to the prison itself. The narrator speaks of the early days of the settlement which, out of the dirt of their soil, built their village. The term "black flower" is used as a metaphor that suggests that the dirt of the soil could only be capable to produce something as dingy and ugly as a prison; a black flower.
..such unsightly vegetation, which evidently found something congenial in the soil that had so early borne the black flower of civilised society, a prison.
Contrastingly, at the foot of the prison stood with much resilience a wild rose bush. The juxtaposition of the bush at the threshold of the prison creates the dramatic irony, where the rose bush is meant vibrancy, uniqueness, resilience, and strength. The narrative explains how the existence of this bush is mysterious, since it seems to have surpassed the passing of time. The narrator gives the rosebush a supernatural origin
whether, as there is fair authority for believing, it had sprung up under the footsteps of the sainted Ann Hutchinson, as she entered the prison-door,—we shall not take upon us to determine.
Therefore, although the prison is figuratively identified as a "black" flower, the flowers at its threshold are meant to represent, as the reading says "some sweet moral blossom" that goes completely juxtaposed to the image that the prison represents to the villagers.
As a woman who, herself, is mysterious, enigmatic, strong, and resilient, Hester mirrors the rose bush that has remained standing for years by the prison door. Similarly, as the most beautiful inmate of the prison she also charmed it with her elegant nature, with her beauty, and with the strange magic that she seemed to bring to it during her imprisonment.