What is the base of a seasoning blend for something like roast chicken?
Without a reference to the specific product, I would assume that it is in fact flour.

It's very common for many chicken (and other meat) recipes to call for dredging in either seasoned flour, or flour followed by an egg wash and spice/breadcrumb mix. Since these commercial mixes obviously have to be completely dry, they are most likely to be seasoned flour.

Check out some recipes for seasoned flour (alternatively called seasoning flour), you'll probably find them to be very similar to the mixes you've been using.