Unfortunately for Blake, he spots a familiar tattoo on the werewolf's arm, revealing that the werewolf was his father. This tragic reveal is a callback and a reversal of the ending of The Wolf Man.
"Even A Man Who Is Pure ... May Become A Wolf When The Wolfbane Blooms And The Autumn Moon Is Bright." The Universal series of The Wolfman Including The Wolfman (1941) and its sequel ...