The Blood Forest is one of the Seven Satrapies.
For over 100 years, Ruthgar shared an alliance with the Blood Forest, and many of both Satrapy's noble families frequently married their children to children of the other's noble families. This resulted in most of the current nobles in Ruthgar having some Blood Forester heritage, and vice-versa. This alliance came to an end approximately a generation before the events of the series, with Vician's Sin, which started and long and bloody war between the two Satrapies, leading to the area where the two share a border to be dubbed the "Blood Plains."
At some point during the reign of Prism Gavin Guile (i.e. after the end of the the War Of The Prisms,) Ruthgar and the Blood Forest went to war again, which is implied to not have been the first time. Gavin intervened and managed to forge a peace between the two Satrapies once again.
Those of Blood Forester descent are known to commonly have "deathly pale" skin, "flaming red" hair, and "cornflower blue" eyes.
Due to the extreme paleness of their skin giving them a great disadvantage in combat, Blood Foresters do not commonly become Blackguards. Liv Danavis stated in The Black Prism that Karris White Oak was currently the only one.
Blood Foresters are known to make and use composite bows that pack as much power as a Yew bow into a much smaller frame.
Given that most of the satrapy is aborneal, giving it its name, it is unsurprising that Blood Forest is the largest exporter of lumber in the Seven Satrapies.