I claim the ornamental design for a blocking tool, as shown and described.
FIG. 1 is a perspective view of the blocking tool;
FIG. 2 is a top side view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a front side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a rear side view thereof, opposite the view in FIG. 4; and,
FIG. 7 is a bottom side view thereof, opposite the view in FIG. 2.
The broken lines depict portions of the blocking tool in which the design is embodied that are not considered part of the claimed design. The long-short lines representing the perimeter of the handle are used to represent the curvature of the handle and are part of the claimed design.