A leap in the dark
Definition 1 of 1
Pro Tip 1/3Risk Implication
The phrase often implies a significant risk or decision with unpredictable results. 
Quitting my job to travel was a leap in the dark.
Pro Tip 2/3Feeling Uncertain
It can express feelings of uncertainty faced when starting something new. 
Moving to a new country felt like a leap in the dark.
Pro Tip 3/3Dramatic Decisions
Use it to describe a bold, possibly dramatic decision without enough information. 
She took a leap in the dark and invested in her dream.