That's bluestain. Its usually doesn't compromise the structural integrity of the stave. I have made bows from staves with bluestain, though it wasn't as dark as the ones in the photo.
It happens in whitewoods that have been cut in summer, when the sap is up, and the humidity is high. That is why most whitewoods cut for lumber is done in the autumn/winter months. Wet wood emits an odor that smells like fishy ammonia, with a hint of sweet alcohol.
Bluestain is not apparent when the tree is first cut. it develops in the days after. If the wood has streaks in it when cut then it is either a normal occurrence in that species, possible insect activity that has healed over, or heartwood rot(usually in mature trees).