"Blackout" and "room-darkening" are often treated as the same thing. They're not — and the difference decides how dark your room actually gets.
Room-darkening
Room-darkening fabric blocks most light but not all — some still passes through or around the fabric. It's enough to take a room from bright to dim and comfortable for sleep for many people, while keeping a softer, less total feel.
Blackout
Blackout fabric is opaque — no light passes through the material itself. The catch is the edges: light still leaks through the small gaps at the sides of any blind. For true darkness you pair blackout fabric with side channels or a fascia that close those gaps.
Which do you need?
For most bedrooms, a good blackout blind fitted close to the glass is the answer. For nurseries, shift workers and home theatres, add channels for near-total darkness. If you only need to take the edge off morning light, room-darkening may be plenty.
FAQ
Does a blackout blind block 100% of light?
The fabric is opaque, but light leaks at the edges of any blind. Side channels or a fascia get you close to total darkness.
Is room-darkening enough for a bedroom?
For many people, yes. If early sun or streetlight wakes you, step up to blackout with edge channels.