The ornamental design for a stadiumblanket, as shown and described.
FIG. 1 is a top front perspective view of a stadium blanket showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a bottom side elevation view thereof;
FIG. 5 is a left side elevation view thereof; and,
FIG. 6 is a perspective view of a stadium blanket being worn by a person.
The broken lines showing a human figure in FIG. 6 illustrate environmental subject matter meant to assist in showing how the claimed design appears when being worn and form no part of the claimed design.