I think you've misunderstood this. The default value of a field is used when you first CREATE a page. If you are UPDATING a page, the default value does not come into play. The flow is assumed to be this: you create a new page, the default values populate the form fields and you save them. After that point, the edit operation is simply displaying the stored values.
When you are outputting values (i.e. printing values in your templates), the default value is meaningless: you are simply printing the stored values. If you stored a value that happens to also be the default value, it gets printed. If you stored some other value, that gets printed. There's nothing tricky going on there, we're just printing the meta_value associated with the given meta_key. Garbage in, garbage out.
Re a blank field: no, that makes no sense. A default value defines a value to use when the field is created, so you can't have both a default value and have a blank value -- it has to be one or the other.
I can understand your confusion because this isn't the only way to handle this -- it's a subtly complex discussion. Consider a couple mind-bending use-cases:
1. Imagine you have existing pages and you add a new custom field X with a default value of Y. When you print a page that uses those fields, what should it print for the custom field X? Should it "lie" and print "Y" when there is in fact nothing in the database yet for that field? If you are searching for posts based on values for that field, how should that work? You may be able to "lie" to your visitors by printing a value that does not exist in the database, but you cannot "lie" to a MySQL query.
2. What if you change the default value of field X from Y to Z. Should this be a destructive edit and CHANGE all values in the database that were formerly Y to Z? If yes, you risk corrupting data. If no, you risk confusing users who are entering/updating data.
So there's really no perfect solution here -- I just opted for the most transparent, simple option that was the least likely to blow things up.