The most common creatures I find in blue cards are Humans and Zombies. I am currently working on a blue-black, or UB, Zombie deck.
The black creature cards are mostly Zombies (my favorite), Vampires, and Demons (some cool cards here). Black cards are all about taking life away from your opponent even if it takes sacrificing one of your own creatures. Black cards have some of the coolest designs, and also some of the coolest names (like Zombie Apocalypse....nice!).
Dementia Bat has an ability that can only be triggered if you cast 4 mana of any type and one swamp land card, which is a hefty casting cost.