The ornamental design for a cordholder for window blind, as shown and described.
FIG. 1 is a top view of a cord holder for window blind, showing my new design.
FIG. 2 is a front view thereof, illustrating an ornamental motif.
FIG. 3 is a side view of my new design, a cord holder for window blind, illustrating, via hidden lines, an aperture through the spindle portion of my new design, for receiving a cord from a window blind.
FIG. 4 is a perspective view my new design, further illustrating an ornamental motif; and,
FIG. 5 is a perspective view my new design, further illustrating the aperture for receiving a window blind cord.