The Dodge and Burn adjustments do not affect areas of pure white (RGB value 255,255,255) or pure black (RGB value 0,0,0). This is by design. Here are some tips on what to expect when using Dodge or Burn.
When you apply Dodge (Lighten) to areas that are almost black, the resulting color and amount of lightness that can be achieved will depend on the RGB value and balance of the area you're working with.
The images above all are almost black, with an equal distribution of red, green and blue. The image on the left has an RGB (red, green, blue) value of 10,10,10. The center image has a value of 3,3,3. The image on the right has the value 1,1,1. In each case, the Dodge adjustment turns the area gray, and the overall lightness that can be achieved is higher when the original RGB value of the image is further from pure black (0,0,0)
In cases where the RGB values are not evenly distributed, the Dodge adjustment tends to expose the underlying color.
In the images above, the left image has an RGB value of 0,10,0, so the Dodge adjustment exposes the green tint. The right image has an RGB value of 0,1,0, and so the effect of the tool is more darkness, although still noticeably green.
The same principle applies when using the Burn (Darken) adjustment on almost-white areas.
Since it's easier to see subtle color tones of lighter areas, the color that results from burning these areas is easier to anticipate than when dodging dark areas.