Testing was done against the same level 58 undead on EPL. Both bloodthirst dealt the same damage whether you equipped Seal of the Dawn trinket or not. I suspect it would share its behavior with KT trinket (Mark of the Champion) and Consecrated Sharpening Stones.
Patch notes from 1.12 showed that this was supposed to be fixed:
Bloodthirst: This ability will now correctly benefit from attack power bonuses versus specific creature types.