Helljumper and Uprising). Blood Line is the story about Spartan Team Black, a team of Spartan-IIs I've never heard of before. They're sent on a covert operation to kill some Covenant when their ship crash lands on a moon. At the same time, a Covenant ship also crash lands. Both sides are attacked by an unknown enemy, which captures members of both teams. So the survivors (both Covenant and Human) band together to rescue their comrades.
Blood Line is also the story of family. Team Black has become its own family through its history of growing up together and working together. On the other side, a pair of Elites are brothers. The strongest one, Ship Master Thon, has always looked after and protected his much weaker brother, Reff. Blood Line is about what both kinds of families will do for each other (and also what family means in both contexts).
Finally, Blood Line is the story of how Iona, the AI of UNSC ship Long Time Coming ended up part of Spartan Team Black.
It's a little hard to follow all of the action, but overall Halo: Blood Line is an excellent read.