Dark Romanticism is a literary trend that combines general elements of Romanticism with supernatural and Gothic motifs to create an atmosphere of what we routinely label as "horror." Much or even most of Hawthorne's fiction conforms to this formula to a degree, though not so much as that of his contemporary Edgar Allan Poe. But "Rappaccini's Daughter" is one of the best examples we can focus upon as typifying the essence of this particular type of Romantic literature.
At the heart of the story is a basic Romantic concept, that of man's striving for something beyond the ordinary, beyond what human effort is normally thought capable of achieving. What we very often see in the works of nineteenth-century writers is an emphasis upon science, unsurprisingly so given that in the real life of that period science and technology had begun to transform the world. Rappaccini's unfortunate daughter is similar to the monster in Mary Shelley's Frankenstein. She is a creation of science, but in this case, as in Shelley's novel, science has gone amok. Beatrice is a magical being, beautiful but poisonous. The juxtaposition of science, man's progressive tendency, with horror is a standard element of Dark Romanticism. So is the combination of genius, in the person of Rappaccini, with human weakness and failure. Though Romanticism concerns itself with man's striving for the impossible, for that which is beyond the bounds of what the religious beliefs of previous ages had imposed upon man, it also overlays this concept with a pessimistic forecast of man's failure due to his own frailty.
So, we have in the story man's attempt to overstep the limits of what had previously been thought feasible, along with an interest in speculative science and in the horrific possibilities to which science can lead when man fails to achieve the positive results he has intended. Finally, the scene of "Rappaccini's Daughter" is remote from us (and from Hawthorne's first readers) in both time and place. The setting is Italy, apparently in the distant past. To nineteenth-century English and American writers, Italy was an exotic, mysterious place in which the "rules" and standards of the Anglo-Saxon world did not apply. The remoteness of the setting enhances the Gothic atmosphere of the narrative, in which Giovanni falls in love with the beautiful but deadly Beatrice. Hawthorne's tale is thus a kind of Dark Romantic version of Romeo and Juliet, an updating of the ultimate tragedy of star-crossed lovers.