The iblock cookie is an opaque data structure that is returned from either ddi_get_iblock_cookie(9F) or ddi_add_intr(9F). These interfaces use an interrupt number to return the iblock cookie associated with a specific interrupt source.
The iblock cookie gives the system information on how to block interrupts. It is passed to mutex_init(9F) when allocating driver mutexes to be used in the interrupt routine. See mutex_init(9F) for more information.