Blood runes are needed in certain combat pay-to-play spells and some other non-combat pay-to-play spells. Below is a list of spells in which Blood runes are used, as well as their respective spellbooks.
Long before 22 April2008, players who achieved 77 Runecrafting would get a message that they can now craft blood runes even though Legacy of Seergaze hadn't been released at the time.
Prior to the Evolution of Combat update, blood runes were used in high-level combat spells in the standard spellbook. The removal of their usage in these spells caused a dramatic fall in their price.
Prior to the Evolution of Combat update, blood runes were used in casting wave and surge spells in Daemonheim. Since they removed the requirement of using blood runes, they now serve no purpose in Daemonheim other than selling them. It is currently unknown what future purpose they may serve.