The ornamental design for a blockstackinggame, as shown and described.
FIG. 1 is a top perspective view of the block stacking game;
FIG. 2 is a top perspective exploded view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a bottom view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a left side view thereof;
FIG. 7 is a back view thereof; and,
FIG. 8 is a front view thereof.
The claim is directed to the collective appearance of the block stacking game set shown in the figures. The components of the set are arranged on the drawing sheet for the convenience of illustration, and the relative positioning of the block stacking game forms no part of the claimed design.