From what you describe, the blackish area and spots may likely be the result of exposure to water in one form or another. Some high tannin woods, particularly oak and walnut, are more susceptible. I would not use anything other than oxalic acid, because it alone will take these kinds of spots out. Ordinary bleach will open the grain and damage the wood (it actually causes the cellulose cell walls to collapse). It should be applied judiciously to the dark areas. Q-tips work great. As soon as it has done its job, rinse immediately with water and dry. I would not apply the oxalic acid until the stock is completely dry, lest you encourage bleeding into the unaffected part of the stock. This stuff is available in small bottles in better hardware stores. I had to resort to this on a treasured .22 hornet rifle with a similar problem around the steel buttplate. It worked like a charm.
