The ornamental design for a blank for a fencepost, as shown and described.
FIG. 1 is a perspective front view of a blank for a fence post showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof, the left side being a mirror image thereof;
FIG. 5 is a sectional view taken along line 5-5 of FIG. 2;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The blank for a fence post is shown with a symbolic break in its length. The appearance of any portion of the article between the break lines forms no part of the claimed design.