Blood Wizard

Blood Wizards were renegade wizards defeated by Kael'thas Sunstrider as he tried to repair the Arcane Observatories around Lordamere Lake in the Misconceptions campaign mission. They were accompanied by other rogue wizards and their apprentices, and used blood magic.

Speculation
The wizards might have been driven mad by desperation after Dalaran's destruction by Archimonde, thus fighting their own allies in bloodlust. Alternatively, they might have been renegade human blood mages, and seeing as they were accompanied by numerous other rogue spellcasters, likely pillaging the magical city.