Nice turning. It always adds to the fun when you find a piece like that.
I have found that to stop the darkening of wood like that I need to seal it with a few coats of Poly or a salad bowl finish like Behlen's or Generals. The latter is a wipe on and easily applied with a piece of good quality paper towel. I use the blue mechanics towels and use gloves to keep it off you fingers. The top coat won't stop it completely but it really slows it down.