None of the answers clearly describe the root of the problem.
Normally in the db you can make a field null=True, unique=True and it will work… because NULL != NULL. So each blank value is still considered unique.
But unfortunately for CharFields Django will save an empty string "" (because when you submit a form everything comes into Django as strings, and you may have really wanted to save an empty string "" – Django doesn’t know if it should convert to None)
This basically means you shouldn’t use CharField(unique=True, null=True, blank=True) in Django. As others have noted you probably have to give up the db-level unique constraint and do your own unique checks in the model.
For further reference, see here: https://code.djangoproject.com/ticket/4136
(unfortunately no good solution decided at time of writing)
NOTE:
As pointed out by @alasdair in a comment, that bug has now been fixed – since Django 1.11 you can use CharField(unique=True, null=True, blank=True) without having to manually convert blank values to None