The ornamental design for a blank for a cardmailer, as shown and described.
FIG. 1 is a front perspective view of a blank for a card mailer, shown with the main flaps in a partially folded configuration and the inset flap in a folded configuration showing my new design;
FIG. 2 is a front view thereof, with the main flaps in the unfolded or flat configuration and the inset flap in the folded configuration, in which the blank is substantially planar;
FIG. 3 is a rear view thereof;
FIG. 4 is a bottom view thereof, with the top view thereof being a mirror image thereof;
FIG. 5 is right side view thereof;
FIG. 6 is a left side view thereof; and,
FIG. 7 is a rear view thereof, with the main flaps and the inset flap in the unfolded or flat configuration.