Well I read the article by Brian Boggs and he details a lot of difficulties with the quebarcho bark. Someone else mentioned tea which is also a source of tannic acid and should work substantially the same way. I haven't tried either of these, but I haven't had any of these problems using the logwood extract and iron oxide mordant combination. Brazilwood is another traditional plant dye that will give black color when used with an iron oxide mordant. The color is not as true black, but seems to have more depth. I apply both of these solutions with a paper towel or sponge, allow to dry overnight, apply the mordant using paper towel or sponge, allow to dry overnight, steel wool to remove raised grain, finish as you would raw wood. Since you've already started with the iron oxide solution you can probably just continue with any of these methods right over what you've done without having to sand back to bare wood to start over.
Water based India Ink or black leather dye will make it black too. I haven't used a mordant with this, but experimenting with a vinegar wash or iron oxide (vinegar with rusty iron or steel wool) would probably be worthwhile.
I've used black aniline dyes, but haven't been satisfied with the depth of color. The alcohol soluble dyes are likely to be less light fast than the water based.
Any of these solutions containing water are going to raise the grain on almost any wood. It will be worse on oak than on some others because of it's open nature. I've found that steel wool deals with this pretty effectively. Sanding will remove the raised grain but will likely remove enough of the surface that you'll lose some of your color. Steel wool is supposed to work like thousands of little knives shearing off the raised grain fibers without removing the surface (it may burnish it some). Keep the steel wool sharp by continually turning it inside out to expose fresh steel wool. Remember that there is only so much grain to raise so rinsing with water and following with steel wool before you begin staining may also help. If you do the water rinse followed by sanding you probably create a new surface from which to raise the grain.
Since most of these methods involve changing the chemistry of the wood surface there may be some variation because the underlying chemistry of the wood is not perfectly consistent. Testing on some scrap is always advisable.
I love George Franks' books on wood finishing. He discusses dyes at length. Check out Adventures in Wood Finishing. It's a nice introduction.
Have fun, Jim
