These shounen-ai stories have similar atmospheres though one's about vampires and the other about supernatural undertakers - a type of shinigami. In both of these shounen-ai stories the love interest is somewhat cold and secretive and forces the somewhat pathetic hero into a partnership for his own selfish reasons. Both partnerships are co-dependent.
These are both shounen-ai vampire titles. Neither are primarily sexual, but in both stories, the vampire takes the submissive role, contrary to what you might expect. Both stories have a vampire hunter who keeps the vampire as a kind of pet. Though in Dia Game they've ended up as a couple, this has not happened so far in Bloody Mary.
Both are about a bond between a human and a cat-ear-hoodie-wearing vampire. Mary and Kuro both have their tormented pasts (though Mary can't remember his) and despite being incredibly powerful, neither of them is inclined to fight. The stories are quite distinct, however and if you're into vampires, Bloody Mary is a good manga for you.
KimiBoku is shoujo while BloodyMary is shounen-ai but the relationship between the protagonists is strangely similar. The main character is totally submissive to their extremely cold love interest. They base their existence on the other person. Both also include action. All characters struggle with unusual powers. Both also bring up questions of mortality and mentalities either bordering on(KimiBoku) or outright(BloodyMary) suicidal.
Both of these are boys love titles in which the main characters start their relationship off when the uke begs the seme to kill them. There's basically undertones of a dom/sub relationship in which the sub is suicidal and the dom controls those urges for them. If that's your kink. BloodyMary is only shounen-ai while Corsair is much more explicit. The settings aren't very similar either.
Both of these stories are in a modern gothic setting. They both center on a partnership composed of a vampire and a holy being. The vampire has to drink the holy being's blood in order to fight. Thus they team up to survive.
Both stories are shoujo!vampire in style. Though one is technically shounen-ai, it reads like shoujo. There's a vampire with principles. There's a human being who forms a partnership with that vampire, feeding it their blood in order to give it the power to protect them both. And the blood sharing is, of course, vaguely sexual.
BloodyMary and Vassalord are both gothic feeling vampire shounen-ai. Both include a main couple composed of an unusually powerful and moral vampire, and a righteous person with powers of their own. Vassalord is more adult, more well-written, and intended for an older audience. I'd say it feels jousei while Bloody Mary feels shoujo.
Simillarities: Same author, simillar character types, unpredictable plot-twists, simillar relationships
The biggest differences lie on the setting, the chapter number (always remember that this manga was planned to be longer but it had to be shorten by the editor's request, so please, don't blame the mangaka for the end) and the really fast-paced end. Bloody Mary is a great manga to read after Torikago Syndrome. And I'd recommend Torikago Syndrome to any Bloody Mary readers as well. Torikago Syndrome and Bloody Mary are the closest works Samamiya Akaza worked on.