Maybe I don't understand your question correctlly, but all posts have to be in at least one category. So it's not possible to show a post that isn't also in a category.

Lets say I have Category A, Cat B and Cat C.
I post to Cat A all the time and I want that to display on the default blog, which it does.

I post to Cat B only once a week, but I don't want that post to show by default with the rest, unless someone clicks on that category.