Use 'I'll be darned!' in casual settings to show you're taken aback or caught off guard. I'll be darned, I didn't expect you to be here!
Pro Tip 2/3
Not for Essays
Avoid in formal writing or speeches; it's too casual and might seem unprofessional. I'll be darned is something you wouldn't find in a business report.
Pro Tip 3/3
Emotional Color
Conveys mild emotion; it's not as strong as shouting or as bland as stating a fact. You fixed it already? I'll be darned!