Hi, I hope this is on topic. When I play for an extended period of time (>30 minutes), my right hand (on the contact side of the index finger from the middle nuckle to the thumb) appears to be stained black. The longer I play the darker it becomes. The finderboard is suppose to be ebony. Is this normal? Could it be something other than the fingerboard? If so is there anything I can do about it? The bass is Upton's Plywood Medio Fino model. Thanks, G.