The ornamental design for a decorative cornerblock, as shown and described.
FIG. 1 is a perspective view of a decorative corner block according to the present invention;
FIG. 2 is a front view of the decorative corner block of FIG. 1;
FIG. 3 is a right side view of the decorative corner block of FIG. 1, the left side view being a mirror image thereof;
FIG. 4 is a back side view of the decorative corner block of FIG. 1;
FIG. 5 is a top view of the decorative corner block of FIG. 1; and,
FIG. 6 is a bottom view of the decorative corner block of FIG. 1.
The broken lines immediately adjacent the shaded areas or the solid lines represent the bounds of the claim, and all other broken lines represent unclaimed portions of the decorative corner block. None of the broken lines form a part of the claimed design.