I claim the ornamental design for a blotter holder, as shown and described.
FIG. 1 is a top perspective view of the blotter holder of the invention;
FIG. 2 is a left side view of the blotter holder of the invention, the right side view would be a mirror image thereof;
FIG. 3 is a top plan view of the blotter holder of the invention; and,
FIG. 4 is a bottom plan view of the blotter holder of the invention.
The broken lines defining the circular features of the bottom of the blotter depict environmental subject matter only and form no part of the claimed design.