What makes a curtain genuinely blackout versus just room-darkening?

Across the communities we read, the single most-cited factor is a dedicated liner or a second dense layer. r/homeownerstips puts it bluntly: no liner means no real blackout. Double-layer constructions and panels with a black backing layer consistently earn the strongest light-blocking praise, while thinner faux-linen panels marketed as blackout tend to only darken a room.