The ornamental design for a window blind, as shown and described.
FIG. 1 is a perspective view of the window blind showing my new design, the broken away area indicates that the window blind has no specified length.
FIG. 2 is left side elevational view of the window blind. The right side elevational view is the mirror image.
FIG. 3 is front elevational view of the window blind, the broken away area indicates that the window blind has no specified length.
FIG. 4 is a rear elevational view of the window blind, the broken away area indicates that the window blind has no specified length; and,
FIG. 5 is a top plan elevational view of the window blind, the broken away area indicates that the window blind has no specified length. The bottom plan view is the mirror image.