Of the ways to do the "black" thing the easiest I've found is to use a black aniline dye (AKA lampblack), alcohol based (methanol). Being in a medium of alcohol, it won't raise the grain. It's fast and predictable, with no waiting, no vinegar, and no steel wool.
You can mix the dye in ratios that work well for the species you have.
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i've done the steel wool/vinegar technique with great success on walnut, oak, cedar, and mahogany. walnut and oak turned out ebony, while african mahogany and western cedar turned out a shade closer to the "espresso" brown that so many people are crazy about.
the shade/reaction depends upon the tannin content of the wood -- i don't think that poplar has a high tannin content and so it may not work. but i'd suggest trying out this technique on a small project just for the heck of it -- it's pretty cool and reminded me of those science experiments we did back in 3rd grade. it darkens a bit initially, but come back in a couple of hours and you'll be amazed at the color difference.
I have tried vinegar/steel wool ebonizing on poplar and have received only a week reaction. Try black food coloring in rubbing alcohol. If that doesn't work as anticipated, try black Parker Quink (a new version of India ink) found in most office supply stores.
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