Romanticism encompasses more individualism and transcendentalism, while Dark Romanticism kind of rejects that and focuses more on the human psychological side and supernatural elements.
With American Romantic authors like Thoreau and Emerson, you see that they heavily emphasize individualism and reject conformity in favor of self-direction. However, with Dark Romanticists - especially Hawthorne - transcendentalism is rejected, and heavier themes like hypocrisy (mostly in religion) and human error are explored. (Edgar Allan Poe is more of the psychological/supernatural side of Dark Romanticism. He explores the darkness of the human mind.)
I hope this helps! :)
