The ornamental design for a playblock, as shown and described.
FIG. 1 is a top front side perspective view of a play block showing my new design;
FIG. 2 is a top rear side perspective view of the play block;
FIG. 3 is a front view of the play block; and,
FIG. 4 is a top view of the play block.
The intermittent broken lines immediately adjacent the shaded areas represent the bounds of the claim and all other broken lines represent portions of the block that form no part of the claim. None of the broken lines form a part of the claimed design.