blessing in disguise - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
A misfortune that unexpectedly turns into good fortune, as in Missing the train was a blessing in disguise, for if I hadn't, I wouldn't have met my future wife. [Mid-1700s]