The ornamental design for a knifeblock, as shown and described.
FIG. 1 is a perspective of a knife block as seen from the upper left front showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a left side elevational view thereof, the right side being a mirror image thereof;
FIG. 4 is a top plan view thereof; and,
FIG. 5 is a bottom view thereof.
The broken lines in FIGS. 1, 2 and 4 of the knife block are for the purpose of illustrating portions of the article and form no part of the claimed design.