An action or strategy that quickly and effectively indemnifies someone or insulates them from a negative consequence.
Derived from "cover your ass," or CYA. (A blanket can easily and completely cover one's ass.)
When Bob's project group didn't meet projections, the programmers got all the blame because of one ass-blanket email Bob sent out criticizing his IT support.
"Why doesn't the boss fire that lazy, incompetent secretary of his?"
"Because if something doesn't get done, he can just blame her. She's his ass blanket."