The Blue Priest was one of Hera's most trusted servants. He was the leader of the Blue Cult and had special command over sea monsters like the Blue Serpent.
When Hera demanded the sacrifice of Deianeira of Troy, her father King Ilus was unable to sacrifice her. She trusted the Blue Priest to keep Hercules from interfering. The Blue Priest tried to stop Hercules from getting to Troy by sending the Blue Serpent to consume him. Hercules managed to defeat the Blue Serpent and did, in fact, make his way to Troy. There, Hercules and the Blue Priest had an intense battle and the Blue Priest was beheaded (Hercules and the Lost Kingdom.
Ares resurrected the Blue Priest who, in turn, resurrected Darphus after Xena killed him. The Blue Priest gave Darphus command over Graegus to assist in Ares' plans to kill Hercules (HTLJ "The Gauntlet").
Later, when Hercules' mother, Alcmene, was being re-married to Jason, Hera realigned herself with the Blue Priest in hopes of using the event to kill Hercules and Jason. The Blue Priest worked with Patronius to hire a small army of mercenaries to attack the wedding party and he summoned the giant sea serpent, Perfidia. Perfidia swallowed Jason and Hercules, who managed to kill the serpent from the inside and the Argonauts managed to defeat the mercenaries. The Blue Priest fled the scene and was pursued by Iolaus and Iphicles. He prayed to Hera to save him and dove off a high wall. He was killed when he landed. Apparently, the goddess was not to forgive him another failure (HTLJ "The Wedding of Alcmene").