Perhaps you are referring to a "directional" transmitting antenna ! They are quite common in both television and FM broadcasting.WJZ,channel 13 in Baltimore uses a directional antenna in order to protect WYOU TV in Wilkes Barre/Scranton.On the FM side,106.5 in Baltimore uses a directional screen to protect a station in central PA.It is side mounted on channels 2,11,and 13 tower in Baltimore. ( A candelabra tower )
I must confess that with 43 years in television engineering,I have never heard of a station using a "signal blocker".