Bloodthistle was was introduced to World of Warcraft for the Burning Crusade expansion as it can only be found at the Blood Elf’s starting zone, Eversong Woods. Bloodthistle can be consumed to increase spell power by +10 for 10 minutes and the consumer will experience a decrease in spirit by -5 for 20 minutes. To cure the debuff, another Bloodthistle can be consumed or by simply casting the mage spell, Ice Block or the priest spell, Bubble.
Here is a map to where to farm Bloodthistle in WoW.
Bloodthistle can only be consumed by Blood Elves and is not used for any recipes so the value is not great. But it is plentiful for early herbalist to train up their herbalism skills. Bloodthistle can be found near buildings. there is maps below for areas to gather Bloodthistle for some spell power boost.