I have used this product to remove dark stains on many different pieces of mid-century teak furniture, which typically has an oil finish, rather than varnish. If your dark stain is under varnish, you may have to strip off the varnish layer before you can chemically treat the stain.

While Oxalic Acid can be purchased as a specialized "Wood Bleach" at woodworker's supply stores, it is also present as an ingredient in a household cleanser that is commonly available here in the United States, known as "Barkeeper's Friend". This is what I used to bleach out all of the iron oxide stains that are shown on this page, often with dramatic results. It is available at most grocery stores, and I have even seen it in Home Depot, in their cleaning section.