I think embossing is recommended because white inks generally don't give you sufficient coverage on black cardstock. Smudging around with a white pencil and white ink afterwards gives a sufficiently "chalk board" look while the embossing makes your stamping easy to see/read.
I'm adding a sample tag so you can see the effects of the different media - white embossing powder for the sentiment with a white pencil "shadow" around it and white ink smudged on the edges of the tag (watermark from the sadly defunct Papercrafts magazine because it's their photography rather than mine on this, they happened to feature it on a blog post promoting a feature in the mag).
