The title of this book is pretty creepy, but it's not just looming there on the cover to freak us out. It's actually inspired by a famous painting called Midvinterblot. In the novel, this is the painting that Eric Carlsson paints and tries to get displayed in the Swedish National Museum. In real life, it was created by Carl Larsson and actually does hang in the upper stairway in the National Museum. Nice work, Carl.
The piece of art shows the sacrifice of King Domalde (renamed in our story as King Eirikr), who allowed himself to be killed to save his people during a famine. The Swedish title literally translates to "midwinter sacrifice," because that's what's going on in the painting. Eric Carlsson actually explains the significance of all this to the snobs from the National Museum:
"Blessed" does not mean what people think it does. In the old tongue it was bletsian and before that blotsian, and before that, just blod. It means sacrifice.
Sacrifice.
To bless means to sacrifice, and in blood. (4.11.13-15)
Basically, the event that sets off the whole story is the ritual sacrifice—this blessing with blood—of King Eirikr. It's a bit mystical and magical makes things get a little wonky. The book also ends with other sacrifice—the slaughter of Eric Seven—a midsummerblood, if you will. These two acts bookend the entire story, so it's pretty fitting that the title reflects on all this blood and blessing.