What is the base of a seasoning blend for something like roast chicken? I'm not looking for the recipies, I'm just looking for the principle behind this idea.

There is a product that I love which is a seasoning blend that you put in a roasting bag with chicken that then goes in the oven for an hour. The sauce that you end up with in the bag is thicker than water but not very thick at all.

The seasoning blend is floury with lots of herbs and spices in it and I'd love to try and replicate it so that I can get the taste without all of the preservatives in the store-bought version.

I guess that I'll have to experiment with the spices, but can anyone tell me more about the floury base? Is this just flour or is there something like corn-starch in these things?
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.