She appears to be molting.. the black "looks" like the feathers coming in and the white stuff is sheath that should still come off. When I pluck chickens, they do seem to have a black secretion inside the feather follicles.. especially from pin feathers.
The skin doesn't look exactly angry like parasites though the feather that are left behind definitely look like hell leading to some general questions.. I see you are in the US.. how many birds in how much space? You say she's the eldest around a year old and others about 8 months.. so Feb hatched, are they laying? What is fed on a regular basis including treats and supplements? Do you know when was the last time you got an egg from this gal? Any roosters or cockerels in your flock?
Sometimes originally broken or pulled or otherwise damaged feathers may not grow back in until molt.. Regarding the earlier missing ones. She is definitely molting now.. so the new feathers should look much better..
Use a flashlight after dark to look at the belly of the bird and below the vent to get your clearest picture of parasite status.. part feathers down to the skin and look for crawlies running away.. some parasites only come out to feed on the birds but don't live on them.. while others are microscopic and will never be seen only their evidence.. In this case you should be looking for "dirt clumps" dried on close to the shaft near the skin.. on the older feathers not the new ones... or angry red skin.. what I see looks mostly pale and normal light pink.. but my eyes don't see everything well and my screen doesn't often help matters.. so look for the clues and tell us what your eyes are seeing.
Molting is a challenging time for birds! Hope she come out of it quickly!
