The ornamental design for a knifeblock, as shown and described.
FIG. 1 is a side perspective view of a knife block showing my new design;
FIG. 2 is a front elevation view thereof, and the rear elevation view being a mirror image thereof;
FIG. 3 is a side elevation view thereof, and the opposite side elevation view being a mirror image thereof; and,
FIG. 4 is a top plan view thereof.
The broken line knives of FIG. 1 show unclaimed environmental subject matter only and form no part of the claimed design. The broken line portions of FIGS. 2 and 3 show unclaimed subject matter only and form no part of the claimed design.