The ornamental design for a window blind stringholder, as shown and described.
FIG. 1 is a perspective view of a window blind string holder;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a rear view of the window blind string holder shown in condition of use with broken line environmental structure. The broken line showing of environmental structure is for illustrative purposes only and forms no part of the claimed design.