The ornamental design for a blank for forming a container, as shown and described.
FIG. 1 is a front elevational view of a blank for forming a container showing my new design, the rear view being a mirror image of the front elevational view;
FIG. 2 is a top plan view thereof, the bottom view being a mirror image of the top plan view;
FIG. 3 is a left side elevational view thereof;
FIG. 4 is a right side elevational view thereof; and,
FIG. 5 is a front, top, and left isometric view thereof.