I'm working on an idea, and I want to use phrasing we all 'get'.

What do we call it when I'm definitely using a system, but it's basically "hidden" (ie, once people decide what to do, I'll handle die-rolls, etc, so you're thinking more about the setting and scenario than the mechanics)?

The most common term is "black box."

Thank you!

I recommend saying 'I roll the die behind the scenes' because I didn't recognise the term 'blackbox'. The words you used to describe what you want were fine I thought

I have heard and usually say "system blind" and specify what system. Or I would if it weren't for the fact that I write so many of my own systems so I can't really run them system blind.