Do blackout curtains actually block 100% of light?

Rarely at the edges. The high-trust testing writeup we read (sleepopolis.com) measured popular panels as blocking 85-99% of light rather than a literal 100%, and community threads on r/homeowners and r/mildlyinfuriating consistently note that the panel fabric itself can be very dark while gaps at the sides and top still leak light. For true darkness most reviewers pair curtains with a wraparound rod, overlapping panels, or a side-mount track.