When the function IsBlank is used to determine if a Combobox has an empty selection, the function always returns true when no selection is made. This also happens when there is a DefaultSelectedItem.
Because of this behavior there's no easy approach to detect if there is a DefaultSelectedItem; the function always returns true if the user did not click the Combobox during his Power Apps session. I would expect the IsBlank function to return false if there is a DefaultSelectedItem.
Proposed solution: if the IsBlank function is used to check if comboboxes / textboxes are empty, and the Default(Selected)Item is not empty, make the IsBlank function return the value false.