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; and,
FIG. 7 is a bottom plan view thereof.
The broken lines shown in the figures are included for the purpose of illustrating portions of the blank holder and form no part of the claimed design.