If you looked at the end grain, did it look like it had little holes, or was the end closed? Red Oak is like grabbing a hand full of straws- it's a series of tubes and if you set a piece of Rad Oak in water, it will wick up into the wood. White oak construction is completely different and it's one of the reasons that it's better for use in wet locations. It also has different chemistry but both have high tannic acid content, which is why yours was black- yours was in soil with high iron content. Iron and ferrous metal scraps have been used for hundreds of years to stain oak. If you want to try this, put some water & vinegar in a glass jar, then put some iron or steel in it. Nails and screws work well. Let it sit for a week, stirring it occasionally. Once the metal has rusted, dip a rag in it and wipe it on a piece of oak and watch the color change.