Maybe this is liberal brainrot, but Blinken is currently 4th in line for the "throne" after Kamala Harris, Mike Johnson, and Patty Murray. If Biden dies, resigns, or otherwise can't do President stuff, Kamala's next. None of the other three seem to have the kinds of medical problems that Biden has, so they'd probably finish out his term.
American politicians are gateways through which lobbyists hand off legislation. Blinken is the one running US foreign policy (watch him in the audience or behind Biden every time Biden speaks publicly, wincing at every fuck-up Biden does that damages Blinken's ongoing attempts to run the empire) and would continue to run it until the new President replaces the cabinet/staff with their own picks... At which point their own picks would be the ones running the empire.
Blinken wrote a paper more than a decade ago about "solving" the Palestinian problem and iirc it's basically a map of what is happening now. I think Sullivan is certainly part of the group making decisions but Blinken is doing most of the foreign policy stuff
I mean officially speaking, the vice president is the acting president when the president is incapacitated (doesn't have to be dead). Mike Pence did that several times when Trump was in surgery under general anesthesia, iirc.