If there are only a few feathers molting out, all the black ones may be replaced by the color they really should be. If there is nothing medically wrong, chances are Buttons might be stressed by something (although stress usually shows as bars rather than entire feathers) or Buttons is overpreening the dark feathers so that their actual color is no longer visible.
Blue feather coloring in some parrots is an optical illusion (Peachie lovebirds in particular) and what you see is a reflection of color from the feathers. Damage the feather and it will appear dull and usually dark grey or black.