Just a memo for Dweeves, because we've already talked about this subject.
For the time beeing in Magmi 0.6.16.beta4, a blank value is considered as an empty value for any attribut.
If you, in update mode to return to a default value for a store view for an attribut, there is no way by Magmi.
So, it could be better to considere that a blank value is for returning or to impose default value and use an other way to put an empty value.
It could be a string like empty or a dedicated string choosen via a csv file.