Our Rabbis have taught: On entering the barn to measure the newly harvested grain one shall recite the benediction, ‘May it be Your will O Lord, our God, that You may send blessing upon the work of our hands.’ Once he has begun to measure, he says, ‘Blessed be He who sends blessing into this heap.’ If, however, he first measured the grain and then recited the benediction, then his prayer is in vain, because blessing is not to be found in anything that has been already weighed or measured or numbered, but only in a thing hidden from sight” (Talmud Taanit 8b).
More succinctly put, as noted on the same Talmudic page, “In the school of Rabbi Ishmael it was taught: Blessing is only possible in things not under the direct control of the eye, as it is said, ‘The Lord will command the blessing with you in your barns’” (Deuteronomy 28:8)
Herein lies the difference between a blessing and a miracle. If one counted sixty dollars in a friend’s wallet, closed it and said, “May God bless you that you have a hundred dollars in your wallet.”…then that person would be asking God to perform an outright miracle. If one looks at the Bible, one sees that miracles are actually few and far between (the redemption from Egypt and the journey through the wilderness being exceptions to the rule). In general, God does not perform outright miracles, because such actions challenge human free will.
A blessing, however, asks that God increase His favor, and that whatever good is already there be increased or hastened. Rather than an outright miracle, such blessings can be perceived as the natural course of events, with a little help.