A rotor blade may generally include a shell forming an outer skin of the blade, with the shell defining a chordwise curvature. The rotor blade may also include a spar cap extending within the shell along a spanwise direction of the blade. The spar cap may be formed from an assembly of pre-cured laminate plates. In addition, the rotor blade may include an interior shelf positioned directly between the shell and the spar cap. The interior shelf may include an outer surface extending adjacent to the shell and an inner surface opposite the outer surface. The outer surface may define a curved profile generally corresponding to a portion of the chordwise curvature of the shell and the inner surface may define a planar surface along which the spar cap extends in a chordwise direction of the blade. The interior shelf may correspond to a pre-fabricated insert for the blade.