When blood is spilled (or with the assistance of a spell in one instance ), a Wild Power can call upon blue fire and direct it for whatever purpose. Like its name suggests, blue fire manifests in the form of a blue power that has a variety of capabilities, ranging from immolating enemies to protecting a Wild Power and its allies. Blue Fire also demonstrated healing abilities, as seen in Witchlight.
Jez is the first in the series to be seen using blue fire, but chronologically Delos is the first Wild Power to do so.
Strangely, blue fire seems to be damaging towards Delos each time he uses it, but other Wild Powers have invoked blue fire without consequences.
Unlike the other Wild Powers, Delos has been calling upon blue fire since childhood, implying that premature and/or sustained use is damaging. This can further be attributed to the fact that blue fire is supposed to be used at the millennium only, as Jez briefly mentions in Huntress.
Strangely, Iliana was able to invoke blue fire through a spell, whereas the other Wild Powers (being non-witches) can only do it via blood-letting.
It should be noted that Iliana may have been bleeding at the time, as she was assaulted by the dragonAzhdeha beforehand.
Blue Fire was implied to be a branch of the witchlight power by Winfrith Arlin. While blue fire is exclusive to the Wild Powers, witchlight is available to all witches.