To keep someone in the dark

If you purposely don’t tell someone about something, you are keeping them in the dark. Similarly, if you’re a dark horse you have a secret which may surprise people. In contrast, if you run something by someone you are telling someone about something and checking they’re ok with it.

Examples

“My boss kept me in the dark about her plans to move to Singapore. She’s leaving the company this Friday and I’ve only just found out!”

“Did you know that he speaks fluent German? He’s a dark horse, I thought he only spoke English.”

“My team is planning on buying new office furniture, but we need to run it by our manager first.”