A blood rune obtained in the Runespan is created by siphoning blood esswraiths, blood pools (with 77 Runecrafting) and bloody skulls (with 83 Runecrafting). This requires one rune essence and awards 73, 146 and 176 experience respectively.
Blood runes are worth 3 Runespan points and can be used to cross greater missile platforms, flesh platforms and greater conjuration platforms. They can also be used to chip soul esswraiths. They will disappear and be converted into points if you leave the Runespan.
If you try to bank any runes or essence from Runespan you get the message "A magical force prevents you from banking this item."