Tannins in woods such as oak react with iron to form black iron oxide. You can see this if you use iron screws in oak - ideally you should use bronze or brass ones. By making a solution of fine 000 wire wool soaked in vinegar for a few days (or longer) in an old coffee jar, and then filtered through a coffee filter to get the gunky bits (actually chunks of rust) out you will get a pale yellow liquid which contains iron acetate. Brush this onto the oak (or spill a drop on a used teabag to test it) and you will get a black stain. I have linked to a Mitch Peacock YouTube video for a demo.