Yes, Cassetted Roller Blinds come with a bottom rail that fits across the bottom of the blind to block the light coming through the gap between the windowsill and the blind’s bottom bar.

Depending on the fitting scenario (top or face-fix), the bottom rail can be fixed behind or in front of the bottom bar – see below.

Top-fix installation

Diagram - Recess Bottom Rail Fitting

Face-fix installation

Face-fix Cassetted Roller Blind-Bottom Rail
Cassetted Roller Blind with Bottom Rail
Cassetted Roller Blind with Bottom Rail
Cassetted Roller Blind with Bottom Rail