I claim the ornamental design for a window blind, as shown and described.
FIG. 1 is a front view line drawing of a window blind showing my new design. The blind is shown to its fully extended length. The head rail, foot rail, and pull cord are shown here in a dashed line for illustrative purposes only and form no part of the claimed design;
FIG. 2 is a top view section of the blind body.
FIG. 3 is a line drawing of the bottom up and front angel view of a section of the blind body.
FIG. 4 is a view of a section of the folding plan (Solid lines represent folds toward you and double lines represent folds away); and,
FIG. 5 is a slightly angled view (bottom right slightly toward the viewer) of a section of the blind body that is stippled to show surface geometry.