A blood trail was a technique developed by the Nightsisters of Dathomir. With this technique, the Nightsister would use her own blood to mark her target (often a slave) and track them through the blood mark using the Force in case they escaped. The shed blood used for a blood trail could not be removed once the trail was set.
Within the covens, the blood trail is placed upon those who have ascended and joined their covenant. This is done during the Trial of Fire before a series of protection spells are placed along the Witch.