Hatenkou Yuugi, Pandora Hearts Recommendations
While I feel calling either of these animes "dark" is inaccurate, they do both share somewhat gothic sensibility (more like loli gothic than genuinely gothic). What could be considered dark about these animes is greatly lessened or broken up by the characters' personalities and a healthy does of humor. I think the characters relationships have several similarities as well. Mainly I think the respective creators had the same target audience in mind and catered to them appropriately.