Total Recommendations: 7
I've only watched episode 1 of DT so this is what I can say so far:
Both have another world-kind of feeling and they have plot and characters that resemble each other. I think they are really related why? Because they both origin from japanese role-playing video game.
Same type of dark space and people with special powers. It's also very exciting, with a mysterious plot and also to mention; on a screen in Durarara!! they show a picture of Darker than black Ryūsei no Gemini.
The main weapon is a gun. They are also looking for money in black lagoon. The art and the the surroundings in black lagoon kind of reminds me of black cat. Altso there is a team of both genders as main characters. I think at least this anime is quite similar to Black cat
This is also Yaoi with the genre of Romance, Comedy and Drama just like in SP. It's recommended and very famous yaoi anime and manga. The first season though is much better I think
As other has said, this is only Yaoi and not hentai but it's very cute(+ someones getting raped) It includes a man and a boy in a relationship with money in play but it's very romantic too.