The ornamental design for a blankholder, as shown and described.
FIG. 1 is a perspective view of a blank holder, showing my new design
FIG. 2 is a front view thereof.
FIG. 3 is a rear view thereof.
FIG. 4 is a left side view thereof.
FIG. 5 is a right side view thereof.
FIG. 6 is a top plan view thereof.
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is an exploded view thereof.
The broken lines shown in the figures are included for the purpose of illustrating portions of the blank holder and/or a blank positioned in the blank holder and form no part of the claimed design.