Slatted roller blinds for windows - do they darken well?

We are considering equipping all windows with Venetian blinds. We think it looks stylish from the outside. But is it practical? Can the rooms be well darkened with them?

Please share your experiences.

Thank you

What exactly do you mean?

Blinds?

Yes, the blinds. Not regular roller shutters.

Or do you mean raffstores?

Are we talking about the inside or the outside? Fabric, plastic, or metal? Is it about darkness or thermal insulation?