The ornamental design for a blank keyblade, as shown and described.
FIG. 1 is a top plan view of a blank key blade, showing our new design;
FIG. 2 is a bottom plan view;
FIG. 3 is a left side elevation view;
FIG. 4 is a right side elevation view;
FIG. 5 is a rear elevation view;
FIG. 6 is a front elevation view;
FIG. 7 is a top perspective view; and,
FIG. 8 is a bottom perspective view.
The broken lines, which define the bounds of the claimed design, form no part thereof. The broken lines showing the head of key is for illustrative purpose only and form no part of the claimed design.