The ornamental design for an opticalblock for an endoscope, as shown and described.
FIG. 1 is a left side perspective view of an optical block for an endoscope in accordance with the new design;
FIG. 2 is a left side isometric view thereof;
FIG. 3 is a right side isometric view thereof;
FIG. 4 is a top isometric view thereof;
FIG. 5 is a bottom isometric view thereof;
FIG. 6 is a front isometric view thereof; and,
FIG. 7 is a rear isometric view thereof.
The broken lines immediately adjacent the shaded areas in FIGS. 1-7 represent the bounds of the claimed design; the broken lines form no part of the claimed design.