Dominic Keating performed an mesmerizing "Blair" in the episode "Helpless (03x12)" in "Buffy the Vampire Slayer".
The guardian of an vampire, Blair was unlucky. The vampire transforms Blair into a vampire too - and so they fought together on the black side and were killed.
"Blair was one of the few characters whom we see transform from a human to a vampire onscreen."
(source: Buffy.wikia.com - Blair)
Photos of the human Blair:
Photos of the vampire Blair:
Sources and more information about this episode with Dominic Keating:
Many greetings from the cold and rainy Germany!