Hi Mary,
'Cannot be blank' properties is set when we want that field to be required field i.e. it cannot be submitted without filling the field
To set Default value for the dropdown box you need to do the following steps
1. Double click the dropdown field that you want to set default value in the data source task pane to view the "Field or Group Properties" dialog box.
2. There is an option to specify the default value.
3. Type the default value, close the dialog box.
4. Now double click the dropdown in the InfoPath design.
5. The dropdown listbox properties dialog opens.
6. In the dataconnection dropdown, select/Add the dataconnection.
7. When u preview the form, the default value will be set in the dropdown
I hope this will help you
Thank you
Madhuri
http://www.ggktech.com