silver lining (n.)

a "bright side" which proverbially accompanies even the darkest trouble; by 1843, apparently from oft-quoted lines from Milton's "Comus," where the silver lining is the light of the moon shining from behind the cloud.

Was I deceived? or did a sable cloud

Turn forth her silver lining on the night?

I did not err, there does a sable cloud,

Turn out her silver lining on the night

And casts a gleam over this tufted grove.

To which Thomas Warton added the commentary: "When all succour ſeems to be lost, Heaven unexpectedly presents the ſilver lining oſ a ſable cloud to the virtuous."