I haven t seen this in Audubon's or Manx, for what it s worth. I think Little would look proportionately shorter-tailed than Audubon's. Undertail coverts are white in Manx, apparently not variable. Top of webs either purple-flesh with black sides to the outer toe and half of upper side of central toe, or pale pink with black markings. Remainder of toes and claws black.
Pink pigments fade considerably in adults at end of breeding season.
I don't think any of that can be construed to be powder blue. How this would be identified in the field is very poorly addressed in the literature. A good time to look at leg color is during take-off. In daylight hours, shearwaters are often encountered resting on the water, typically in small rafts. Sometimes they shrug to rearrange their feathers and may paddle their legs in the air as they do so.