into the blue
Completely gone or disappeared; entirely out of sight or reach; without a trace. Usually preceded by "vanish" or a similar verb. The brutal dictatorship was so mercilessly efficient that anyone who stood up against it soon vanished into the blue. I have no idea where my keys have gone. They seem to have disappeared into the blue!
into the blue
see under into thin air.
into the blue
At a far distance; into the unknown: spontaneously take a trip into the blue.