The Gemara in Taanis 8b says:
"Ein Berachah Metzuyah Ela b'Davar ha'Samuy Min ha'Ayin" (blessing is found only in an item which is hidden from the eye)
One who enters his granary to measure it should pray that his work receives blessing; once he has started to measure it, he blesses Hash-m for sending blessing upon it.
But if he measured it and said the Berachah afterwards, it is in vain, as blessing cannot happen to items measured, only to those things concealed from the eye.
What exactly is being asked with this blessing?
If the amount is fixed, what effect does the blessing have?
Is it then asking for some kind of miracle?