This is the continuation of the Anime: Blood - The last Vampire.
As far as I know, there is only 1 volume of this Manga, and I doubt there will be a second one.
IMPORTANT: It will greatly help, but not necessary, to watch the Anime first to understand the manga more. You'll get more depth and understanding in the manga, but since the manga was based on the anime, I suggest to watch the show first.
Saya is a vampire-slayer, and is working for the government of USA, but is now stationed in Japan cause of some incidents there. Here the work begins to hunt down a gangleader, but the gangleader has another unknown superior, and Saya have something in common with this unknown person...
I can't say too much of this to reveal the storyline, because this is only a 1-volume Manga...
DISCLAIMER: If you like this, go and buy it to support the authors and their work. Your help makes the authors to continue their work;)