"Payment? ...You've got to give the door something?"
"Yes ...Blood, if I am not much mistaken."
-- Harry and Dumbledore in the sea cave (HBP26)
Blood Magic is a magical effect created by an unknown spell whereby a sacrifice of blood is required to open a portal. A wizard seeking passage must offer some of his own blood on the portal to make it open (HBP26).
References from the canon
- Voldemort used blood magic to seal the entrance to the Sea Cave where he hid his locket Horcrux. Dumbledore cut his own hand to produce the blood needed and sprinkled it on the rocks (HBP26).
- Regulus Black and the House-elf Kreacher had figured out the Blood Magic entrance years before when they replaced the true Horcrux with a fake locket (DH10)
From the Web
"Blood" on Harry Potter Wiki