Story: The idea is new in terms of the actual plot itself, but the main themes in it aren't exactly unfamiliar. The idea of people pacting together to gain extra power is all over the place, e.g. Mahou Sensei Negima. Fairly quickly you're drawn into the story by its intensity, the rapidity of its advancement and the characters who are well casted (even the ones which you hate). A 9 for story because I liked it, although this was the first series I've watched (other than the CardCaptor Sakura series, excluding the second film) where I've yearned for an alternate ending because it left me feeling unsatisfied. If you're someone who has to have an excellent ending then heed this warning: You will be disappointed.
Animation: The animation wasn't exactly a masterpiece; in terms of raw quality I would have rated it somwhere around 7; however, for the series' themes and battles the style fits snugly. The only downside is that in some of the battles the reuse of animation was noticeable, but just about every anime is guilty of this and Kurokami kept this to a minimum. I gave this a 9 for its suitability and general quality.
Sound: Again, the music wasn't exactly a masterpiece. It's not a soundtrack I'd rush out to buy like the CLANNAD or Pandora Hearts OSTs, but it does its job well to the point that it blends in to atmosphere rather than being distinctly noticeable. In my opinion, if you don't notice the music because it's woven into the events unfolding then it has been done skillfully and effectively. When the music needs to be heard, it is heard. Thus, I gave it an 8 as it was good, but not exceptional.
Characters: The characters were well suited to their jobs. From Keita's detached personality which develops to reveal his true self to Kuro's consistently cheerful and happy attitude which can become serious in the appropriate situations. A 9 for the characters because, while not as entirely unique as you may expect, we didn't have a classic Tsundere character as such which gives it a slightly fresh air, for me at least.
Overall: 9.5. It would be a 10 because, for the whole anime up to the last episode, I was engrossed in the plot, I adored the characters and the battles. The last episode was possibly the biggest let down in anime that I've yet seen: An episode which is almost entirely re-done scenes from earlier on with a few extra bits and monologue from the two main female parts, then an epilogue that made me want to write very angry e-mails to the writers.
It's worth a watch and you will love it, and then cry in anger at the end.