I personally really liked Darker Than Black. It didn't pull me in like some other series but it was definitely different and enjoyable enough to keep me watching.
As far as violence is concerned, I found it to be very tame overall. In fact, if someone asked me if this was a violent series, I would be inclined to say no, for the most part. (This is coming from someone that used to suffer from haemophobia, mind you).
I recommend it, like I do most BONES anime. Top notch
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