The dark spots are actually dirt/oil absorbed into the damaged surface when the timber was bruised. In order to remove the dirt you need to raise the pores of the crushed/damaged area.
Only steam will do this, and if used with a detergent, will remove the marks, at the same time allowing what little elasticity left in the timber to straighten, if the damage is old, the timber can only lift slightly, but usually enough to allow the pores to open.
This envariably leaves you with raised pores and a slightly rough surface than can be smoothed by normal means, then clean linseed oilicon will need to be applied to bring the whitened timbers colour back to normal.
Good luck.