You might try Oxalic acid. However, IF it removes the stain, you might have the opposite problem of the spots being lighter than the surrounding leather. Try it in a small, inconspicuous area to see. Here is a link to a thread about it.
Oxalic Acid And Cleaning Leather - Dyes, Antiques, Stains, Glues, Waxes, Finishes and Conditioners. - Leatherworker.net