When the woodcock flew up in its flash of reddish brown, his heart had knocked hard. As they rode he was aware of it, each beat a heavy wing-beat; as the bird found the concealment of trees, its tracing of feathers inked out to black.
Bring Up the Bodies by Hilary Mantel
What does 'inked out to black' mean?
To ink out a word is to write or scribble over it with the nib of the pen until it becomes illegible.
The author of these handwritten missives has inked out a word which can only be guessed at from the descenders and ascenders that remain visible.
Thus "its tracing of feathers inked out to black" would mean "its tracing of feathers having become indistinguishable (like letters which had been inked out)".
