1.   If so, removing the stains would be almost impossible unless you use a wood bleach.

2.   If so, you will have to use a wood bleach that is sold for this purpose in most paint stores and home centers.

3.   Sanding will usually remove the dark stains, but sometimes you may also have to use a wood bleach on the stains that still remain.

4.   Then use a wood bleach to lighten the wood before re-staining it in the color you want.