The ornamental design for a duck blind, as shown and described.
FIG. 1 is an isometric view of a duck blind in a closed position showing my new design;
FIG. 2 is an isometric view thereof in an open position;
FIG. 3 is a top view thereof;
FIG. 4 is a front view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a rear view thereof; and,
FIG. 8 is a bottom view thereof.
The broken line showing of a duck blind located on the right side, camouflage material, plants, ground, a person and an interior rectangular space in FIGS. 1 and 2 is included for the purpose of illustrating portions of the duck blind that form no part of the claimed design.