The ornamental design for the portion of fan blade, as shown and described.
FIG. 1 is a perspective view from below of a portion of fan blade in accordance with my design;
FIG. 2 is an enlarged bottom plan view of a portion of fan blade in accordance with my design; and,
FIG. 3 is an enlarged bottom plan view of a portion of fan blade in accordance with my design.
The broken line showing of the ceiling fan blade holder is included for the purpose of illustrating environmental structure and forms no part of the claimed design.