The ornamental design for a blind casing, as shown and described.
FIG. 1 is a perspective view showing the front, top and left side of the blind casing of my new design;
FIG. 2 is a perspective view showing the front, top and right side thereof;
FIG. 3 is a perspective view showing the right side, top and front thereof from a different angle;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a front view thereof; and,
FIG. 7 is a rear view thereof.
The break lines shown in FIGS. 1-5 symbolize breaks in the length of the article in which the claimed design is embodied. The appearance of any portion of the article between the break lines forms no part of the claimed design.