Consider:
WHERE DVDColor = "Red"
That returns only records with "Red"
WHERE DVDColor <> "Red"
That returns every record except those with "Red"
WHERE DVDColor Is Null
That returns only records where field is Null
WHERE Not DVDColor Is Null
That returns every record except those where field is Null.
If you still have a blank row in the combobox list, there must be an empty string in the field.