On the left side of the screenshot below, you'll see your category list. This list contains a variety of starter categories. You can even add your own categories using the Board Organizer (more on that later). On the RIGHT side displays all of the boards contained under that category:

By clicking on one category, all of the boards under that category will be selected. You can select and de-select boards within a category as you like.

Simply select the categories you want to blast to, and click on NEXT to start blasting.

If you don't see any boards there, that means you have selected a different database, and there are no boards in your database.

To dive deeper into the contents on your database, click on the "Organize your Boards" button to launch the Board Organizer:

 Then click OK, now you should see that boards in the "default" database.