What is the meaning of "casting a blind eye"?

The usual idiom is to turn a blind eye, which means:-

Fig. to ignore something and pretend you do not see it.

The usher turned a blind eye to the little boy who sneaked into the theater. How can you turn a blind eye to all those starving children?

Probably the most famous instance of this was Admiral Nelson at the Battle of Copenhagen, who is said to have disobeyed Sir Hyde Parker's order to withdraw by holding his telescope to his blind eye to look at the signals, and then claiming not to have seen them.

The verb cast is often used to mean

to direct (a glance, attention, etc): cast your eye over this

To cast a blind eye is to not see something.

The phrase has a slightly ironic flavor since casting an eye is a deliberate act. So casting a blind eye suggests deliberately not seeing something that you do not want to see.