Green’s Dictionary of Slang

blanket v.

1. (N.Z. prison) to assault and beat after covering the victim’s eyes with a blanket or pillow-case.

[NZ]B. Payne Poor Behaviour 116: [M]ost of the other creeps are regularly blanketed and beaten with iron bars, while their beds are filled with piss and shit and some are scarred for life.
[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 20/1: blanket v. 1 to assault somebody.

2. (N.Z. prison) to provide an alibi or excuse.

[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 20/1: blanket v. 2 to provide an alibi or excuse for another person, to 'cover up' for him.