The ornamental design for a seatingblock, as shown and described.
FIG. 1 is a front, top and right-side perspective view of a seating block;
FIG. 2 is a right-side-elevational view thereof;
FIG. 3 is a top-plan view thereof;
FIG. 4 is a back-elevational view thereof;
FIG. 5 is a bottom-plan view thereof; and,
FIG. 6 is a front-elevational view thereof.
The right and left sides of the seating block are symmetrical and, therefore the left-side-elevational view and right-side-elevational view are the same.