From Longman Dictionary of Contemporary Englishleap in the darkleap in the darkRISKsomething you do without knowing what will happen as a result → leap
Examples from the Corpus
leap in the dark• Critics also worry that compassionate conservatism is a leap in the dark without any empirical evidence to back it up.• The man who hopes to be Chancellor next week can not surely be preparing a leap in the dark.• Ten minutes later - another leap in the dark - he offered the appointment to Churchill. 