Looks like old dried wood to me. It could be mold but not a rot

If unsure, let it take some hours of direct sun exposure to reach high temperatures for most molds

Could be fungal discoloration from being eaten by bugs. Might look for worm or bug damage in/around that dark spot to confirm. Wood can also turn that color when in contact with metal for a long time, but I'm pretty sure that's fungus. Should be fine to use as a walking stick as long as it's not soft and spongey at that spot.