Keep something dark
Definition 1 of 1
Pro Tip 1/3Privacy Matters
Used when information is sensitive and revealing it could have personal or professional consequences. 
It's best to keep your medical history dark at work.
Pro Tip 2/3Connotation of Mystery
Implies something mysterious or intriguing, can make others curious about the hidden information. 
She kept her past dark, which made everyone more interested.
Pro Tip 3/3Trust Essential
Used in contexts where trust between the person keeping it dark and the confidant is vital. 
I knew I could keep the secret dark with him; he never gossips.