The ornamental design for a laddercord for window blind, as shown and described.
FIG. 1 is a perspective view of a ladder cord for window blind;
FIG. 2 is a front view thereof, the rear side being a mirror image;
FIG. 3 is a right side view thereof, the left side being a mirror image;
FIG. 4 is a perspective view thereof shown in a released position in use on a window blind; and,
FIG. 5 is a perspective view of the ladder cord for window blind of FIG. 1 in a lifted position in use on a window blind.
The top and bottom ends of the ladder cord shown in broken line depict portions of the ladder cord for window blind that form no part of the claimed design. The broken lines showing a window blind in FIGS. 4 and 5 depict environment and form no part of the claimed design.