Blue Exorcist is a
colorful anime. Meaning it has not only beautiful graphics, but also variety of unexpected events and "sides" of characters.
The anime centers around two brothers - the older one who inherited the power of Devil himself, Rin Okumura, and the little one who seems to be another quite citizen, Yukio Okumura. Both of them were adopted by a very powerful exorcist, Shiro Fujimoto, who tried to raise them as good as he could.
However, one day a certain guest visits Shiro's church and boys' lives take another, adventurous direction. I really don't want to give any spoilers, because Blue Exorcist has surprises for you right from the beginning.
All I can add that it has a lot of comedy along with action. I can't tell that these genres are combined as perfectly as in Kuroshitsuji (Black Butler); however, this fact doesn't really make the anime less entertaining.
So, if you are into fights between humans and demons, don't waste your time searching for a better series. Blue Exorcist is great!