The ornamental design for a blanket, as shown and described.
FIG. 1 is a front elevational view of my new blanket; and,
FIG. 2 is a back elevational view of my new blanket of FIG. 1.
The broken lines in FIGS. 1 and 2 depict the boundaries of the claimed design.
Additionally, FIGS. 1 and 2 include a set of broken lines depicting the image of a baby which are portions of the blanket that form no part of the claimed design.