The first thing I did when I got it, still had manufacturing black stuff on the back. (goodereader.com)
Soy sauce may seem like the least of your worries, but that innocent looking black stuff almost always contains wheat and may or may not mention it on the label. (ericbakker.com)
You can also put some white paper down there and see how much black stuff is coming out. (mechanics.stackexchange.com)