A ManyToManyField isn't itself a column in the database. It's represented only by an element in the joining table, which you have here defined explicitly as AnnotBlast (note that since you're not defining any extra fields on the relationship, you didn't actually need to define a through table - Django would have done it automatically if you hadn't).
So to add data to your models, you add data to the AnnotBlast table pointing at the relevant Blast and Annotation rows.