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.
Darth Caedus used this Force technique on his sister, Jaina Solo during the Second Battle of Roche in 41 ABY, with the intent that it would lead him to the hidden Jedi base. It was noted by Jaina as feeling acidic. Later, by 43.5 ABY, Luke Skywalker had also learned this technique; he used it to track the Sith apprentice Vestara Khai after their duel on Sinkhole Station.