Blundell started on the series as a producer for Call of Duty 3, but he's best known for managing its long-running zombies mode. He headed up both the undead mode and the main story for Black Ops 3, and focused on zombies for Black Ops 4. There's little doubt that he played an influential role in the Call of Duty series when he took his co-head role in 2016.
His exit comes at a less-than-ideal time for the first-person shooter franchise. Treyarch is believed to be working on Black Ops 5 for this year following turmoil at Raven and Sledgehammer, who were reportedly set to release a Call of Duty game this year but were demoted to support duties after rivalries and departures caused chaos. If so, Blundell's departure will throw more uncertainty into the mix. While Treyarch may still be in good hands, it probably didn't want to lose leadership at a moment like this.