Is the rind missing in those areas? If so, why is it discolored? I'm thinking maybe the rind was rubbed off while the bamboo was still alive, or very green, and then it oxidized in those areas. That alone would convince me to use a different piece. It may also be bruised under there. My view is... making a bow is an investment of my time and energy, so I try to make decisions that optimize my chances of success. Even an apparently perfect-looking piece of bamboo can raise a splinter, let alone pieces that are obviously damaged or 'seconds'. Did you get it from a supplier who is familiar with the needs of bowyers?