Black Walnut Tree -- Borer?
Yes, this sawdust is from insect borings. There are a couple of insects that can bore into black walnut, though they are not common. We cannot make a diagnosis from the photos. Could be flatheaded appletree borer, leaopard moth borer, possibly ambrosia beetle borers.http://extension.umd.edu/hgic/topics/borers-trees
See if you can find the bore holes higher on the tree, and send us photos (close and clear).
Borers are often attracted to stressed trees. The main thing you can do to help your trees is to not stress them.
We notice that your tree is planted so deeply, or had soil added at the base, so that the tree flare is not visible. This can stress a tree. You may want to scrape away some of the soil around the base and expose the flare (where the trunk widens). See here for more info about tree issues: http://extension.umd.edu/sites/extension.umd.edu/files/_docs/programs/hgic/HGIC_Pubs/Landscape/HG201...