This seems to be two questions:
1. The repairs should be accomplished as recommended in ICRI Technical Guidelines: the area to be repaired should be chipped out to sound concrete, sand­blasted or waterblasted to remove any bruised material, dampened to a saturated surface-dry condition, and patched with a reputable and known repair mortar.
2. ACI 506 defines “shadow” as any porous areas behind an obstacle, such as reinforcement. Proper shotcrete application by an experienced nozzleman with adequate equipment and crew will create shotcrete sections that have minimal shadowing and voids. However, sometimes upon stripping of the backside forms there is a noticeably darker coloration of the concrete surface immediately behind reinforcing bars. This darker coloration often does not have porous areas, but is an indication that the reinforcing bar is close to the form and, although good, dense material has been placed, it has a slightly higher cement paste content and thus appears darker on the surface. Sounding the area of discoloration should identify any porous areas that should be repaired, as mentioned previously.
It should be noted that this is not unique to shotcrete. Cast concrete that is not properly placed and fully consolidated often exhibits voids and rock pockets that need to be repaired upon stripping of the forms.