If you liked the Black Lagoon anime, the Anime-Planet community thinks you'd like:
22-year-old Ryouta is an unemployed good-for-nothing who spends his days living at home and playing Btooom, a wildly popular online game that's sold over three million copies worldwide. Alongside other teammates, Ryouta battles it out with others using powerful bombs and sonar that can reveal hidden enemies, and his dedication has earned him the title of Japan's top player. But soon, Ryouta's pastime becomes a terrifying reality, as he and many others are dropped onto a tropical island and forced to play Btooom – but this time for real. Now, he and the rest must fight to the death by whatever means necessary, as the only way to win this deadly game is by killing seven of the other players.
If you like Btooom! you will probably like Black Lagoon as they both have unfair elements that force the main charachter to fight to survive. However, Black Lagoon is much more darker and extreme.
In the distant future, mankind has mastered space and spread empires across the galaxy. While many choose to colonize distant planets, others choose to remain amidst the stars, ultimately giving rise to a new brand of humanity known as the Abh. Both genetically and culturally different from their Earth-dwelling peers, the Abh soon find themselves engaged in a bloody war that rages across hundreds of planets and set out to restore peace by means of conquest. Enter Jinto, a nobleman and ambassador of the recently acquired Hyde system whose duty is to represent his peoples' interests and rule on the Abh's behalf. In order to be officially coronated to this position, a cold-but-beautiful Abh princess named Lafiel arrives at Hyde to escort him back to the empire's capital. When they are suddenly attacked by an anti-Abh liberation front, however, the festivities are cut short, and the two must flee for their lives against all odds.
Both anines focus on the relation between a female protaginst with fighting/military skills and a timid male protagonist.
If you liked Black Lagoon solely because of the action and humor, Crest of the stars is not necessarily for you, but if you enjoyed the relationship between Rock and Levy, you will definitely like Crest of The Stars.
If you liked Crest of the Stars, you will also like Black Lagoon. It also has this romance and action, but to top it off, Black Lagoon also has very funny moments!
Maia Mizuki has just had all of her hopes and dreams shattered. Not only did she fail her entrance exam to the Ocean Agency, a prestigious marine law enforcement agency, but she’s also been left penniless, homeless, and alone. She soon finds herself caught in a gun fight between two scantily clad women and a pair of bank robbers. The women are from Nereids, a private detective agency that will do anything if the price is right. One thing leads to another and Maia takes a job working for Nereids, where she"ll receive room and board. Her jobs range from collecting rent to saving pets to capturing criminals. Now she just has to balance her life of "fighting crime" with her dream of one day being a part of the Ocean agency!
Black Lagoon and Daphne in the Brilliant Blue are both series quite aquatic in nature revolving around outfits providing services along the lines of mercenaries, couriers, or bodyguards. Daphne is arguably more ridiculous, but they are both to some degree lighthearted.
Have you ever felt like the world would be a better place if certain people weren’t around? Such grim daydreams might occur when watching the dismal daily news, but on one fateful day, Yagami Light finds that these daydreams can become reality. By pure happenstance, he comes across a black notebook entitled "Death Note", whose text within states that whoever's name is written on its pages will die. With the aid of the death god Ryuk, Light takes it upon himself to rid the world of its corruption, ushering in a new era of purity one death at a time. But as Ryuk foretells, Light's actions will not go unchallenged...
Both shows have several groups plotting against each other. The twists and turns in the crime world that these shows feature are extremely interesting and any fan of one would be a fan of the other.
In a city laced with crime, four individuals face their own personal trials. Mihai, a middle-aged hitman, seeks closure with an old student; Badou the information dealer learns a little too much; Naoto, an orphaned girl, lives for revenge against her parents' killer; and Heine, the result of underground genetic experiments, hopes to save another like him. Each must find the strength to overcome their dilemma and continue to live in this tough world...
If you found one of these show's (very stylized) gangster action coupled with more introspective parts where the characters backgrounds and motivations are explored appealing you might want to check out the other one.