The ornamental design for a blanketholder, as shown and described.
FIG. 1 is a top, front, and left side perspective view of a blanket holder showing our new design;
FIG. 2 is an additional perspective view thereof, the broken-line disclosure of rolled blankets being shown for illustrative purposes only and forming no part of the claimed design;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a left side elevational view thereof, the right side being a mirror image of that shown;