Blood+ was truly an interesting anime - it requires patience but once one starts to get accustomed with the pace and characters, it is an amazing anime.
Story wise Blood+ starts slow. It takes time to develop, to let the elements of the story, for the characters to interact naturally rather than forcing them into a swirling bog to just push the story ahead. And it starts with the basic anime form: The basic elements are put forth - Chiropteran and Saya. One can assume that with the progression of stronger foes and conflicting feelings, there is nothing more to the anime than the main character who is initially weak growing stronger to face all foes.
However as the story progresses one finds out that it is not so. There will be additional elements which do not appear as a new boss might when the previous one dies. They actually develop a very intricate plot. And it's approach is slow. It takes four to five episodes to let the character figure his/her own weakness and then stabilize as they would in a real world scenario. Claymore on the other hand (ignoring how its story progresses) manages to do all that in the first episode. Gantz takes up to three episodes for that.
While this progression initially feels slow in contrast to animes like Mirai Nikki or Sword Art Online it becomes a positive because unnecessary character development and huge increments in power is avoided and there is time for small plot details to be handled. The story starts taking proper shape around the tenth episode and for a fifty episode anime that isn't too slow. There are various side stories and endeavors that combine together and connect (sometimes too big a coincidence) and keep the viewer up to date with all the characters. It is also a clever way of telling parts of the story from another perspective.
So all in all it has a very complex and well thought story and yes - if I want I can sit down and find faults like "did they close the gate or not" but it is a good thing that in the entire anime such frivolous events are taken for granted. Meaning thinking about the guards changing shift is not our business.
The art style isn't that bad for its time. The backgrounds are good and at times rather beautiful in terms of vibrant colors in contrast to dull underground moments. There isn't a fixed atmosphere, one will be able to see light slice of life like moments and one will see bloody sequences. And neither feel out of place.
The character designs are mixed - Saya and Haji and Solomon (regardless of his stereotypical prince look) are drawn well - they have their physical traits that show detail in their art work. However there are a few characters (Kai for instance) that just seem like a few lines under a scruff of hair at times. It isn't that distracting but at times I feel Kai lose expressions or originality other than his hair. However this is being critical - the characters are more or less consistent with real-life proportions and we have one piece as well in which some character faces are very basic (ignoring the unrealistic proportions they at times have).
The soundtracks in Blood+ Mostly left a positive impression on me - they had a nice gist to them and they were timed well. Emotions and scenario's are well expressed through suitable sound tracks. Fighting sequences were well complimented and so were the slice of life moments - much like the art they managed both sides well.
Character development was a big positive for me in this anime. Yes, we do have the hero that at times is swayed and needs memories and friends n family to go through tough times but there is quite some work put in the development of different characters - some subtle characteristics that one notices afterwards and also traits that make them less predictable in the anime realm.
Saya - as an instance did portray the typical shounen main hero (Ichigo for example) in which they are people who preach justice all the time and are against fighting (a better example would be Yuuto from Omamori Himari) however as the story progresses she gets stronger and mentally more mature and sees the situation from a different light - something that doesn't happen in a day or so.
Kai, as much as I find his art style in the start not too good - is a good character, impulsive and daring however he isn't the guy you would always expect to go haywire due to his personality, who butts into everything for the sake of protecting everyone. Wow... that kind of reminds me of Kato in Gantz... although he was also properly crafted, a little less multi faceted as he was.
Enjoyment wise it will take the anime about ten episodes to kick in - but then the believable characters, the complex plot, all of these elements combine to give what I'd say is one of the best animes I have seen. It is well balanced - not diving into a world of needless action, nor is it slow enough to bore you.
Thank you for reading and feedback is always appreciated.