(to be) a blessing in disguise: A good thing that you don’t recognize at first.
"The hotel is full tonight; we will need to find a new place to stay."
"Maybe it's just a blessing in disguise; I've been looking forward to trying a new place anyway."
Cambridge Idioms Dictionary: http://www.cambridge.org/us/esl/catalog/subject/project/item405031/cambridge-idioms-dictionary-2nd-edition/?site_locale=en_US
