The ornamental design for a block, substantially as shown and described.
FIG. 1 is a front perspective view of a block in accordance with the present invention;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
Any shading or hatching are not features of the design and are merely utilized to illustrate surface contours of the design in the drawings. The broken lines depict portions of the block that form no part of the claimed design.