The ornamental design for a blankethammock, as shown and described.
FIG. 1 is a top perspective view of a blanket hammock showing my new design;
FIG. 2 is a top view of the blanket hammock;
FIG. 3 is a bottom view thereof;
FIG. 4 is a side view thereof;
FIG. 5 is a side view opposite that shown in FIG. 4;
FIG. 6 is another side view thereof; and,
FIG. 7 is a side view opposite that shown in FIG. 6.
Those portions of the figures that are presented in broken lines merely illustrate the environment of the design and are not part of the claimed design.