The ornamental design for a cardboardblank, as shown and described.
The sole FIGURE is a plan view of the cardboard bank showing our new design.
The broken lines, the area within them, and all areas adjacent to the shaded planes depict portions of the cardboard blank that form no part of the claimed design. Solid lines located on internal edges of the cardboard blank in the FIGURE and having a lighter line weight than the solid lines located on perimeter edges depict panel crease lines. Solid lines located on internal edges of the carton blank in the FIGURE and having the same line weight as the solid lines located on perimeter edges depict cuts or partial cuts in the cardboard blank.