"... one finds blackthorn wands among the Aurors as well as among the denizons of Azkaban."
--Mr. Ollivander (Pm)
A wand wood, known for its dark bark, large sharp thorns, and dense, stiff, spiny branches (Pm).
- Ron "liberated" a blackthorn wand from a Snatcher who had tried to capture him (DH19). Harry used the extra wand since his was destroyed while escaping Nagini, but the blackthorn did not work well for him (DH20)
- According the Ollivander, a wand of this wood "has the reputation, in my view well-merited, of being best suited to a warrior." He mentions that the blackthorn bush produces its sweet berries (sloe berries) only after a hard frost as evidence the the blackthorn wand needs to pass through difficult times with its owner before truly bonding (Pm)
- According to legend, Sir Cadogan carried a blackthorn wand with a troll whisker core (Pm).
Commentary
Etymology
The common name "blackthorn" is due to the thorny nature of the shrub, and its very dark bark (Wikipedia).
Notes
The Irish name for Blackthorn is "Straif" - meaning "strife," perhaps because of the tree's dangerous thorns. It was believed the blackthorn was guarded by spiky fairy-folk known as the Lunantisidhe, or "moon fairies." Leprechauns were said to make their fighting clubs from blackthorn, and witches their magic wands. In 1670, a Major Thomas Weir of Edinburgh was suspected and burned for witchcraft because he carried a blackthorn walking stick. source: IrelandCalling
Blackthorn is traditionally the wood used for the shillelagh, the Irish fighting stick. 
More legends and lore.