The ornamental design for a windowblindslat, as shown and described.
FIG. 1 is a perspective view of a window blind slat showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The break lines shown in FIGS. 1 and 4 -7 symbolize breaks in the length of the article in which the claimed design is embodied. The appearance of any portion of the article between the break lines forms no part of the claimed design.