Definition

A transmission from one station to another station in circumstances where two-way communication cannot be established but where it is believed that the called station is able to receive the transmission.

Source: ICAO Doc 4444 PANS-ATM

Description

Blind transmissions can be used when providing air traffic service (ATS) in case of radio communication failure. When the the controller (or FIS officer) expects that the loss of communication is one-way (i.e. the pilot can hear messages but is unable to respond), they may provide information (e.g. about an aerodrome, weather, airspace restrictions, etc.) or in some circumstances issue clearances and instructions. 

Pilots can use blind transmissions in various situations, e.g.: