Based on some testing, it looks like searching for assets using a parent category doesn't return the asset. Using the Category Navigation portlet to filter results works fine when using it to sort by parent category. Here are the steps I took to test.
1. Created vocabulary called Fruit
2. Created the following categories and sub categories
3. Created Blog Entry 1 and used category Red
4. Created Blog Entry 2 and used category Green
5. Created Blog Entry 3 and used category Banana
6. Clicking on Apple in the Category Navigation portlet returns Blog Entry 1 & Blog Entry 2 in the blogs portlet
7. Clicking on the Apple in the Category Navigation portlet returns Blog Entry 1 & Blog Entry 2 in the Asset Publisher
8. Searching for Apple in Blog Search returns nothing
9. Searching for Red or Green in Blog Search returns the Blog entry
10. Searching for Apple in Search portlet returns nothing
11. Searching for Red or Green in Search portlet returns the Blog entry.
LPS-5898, it seems like #8 & #10 should return Blog Entry 1 and Blog Entry 2.