Many Orthodox have the practice of blessing grapes or fruit at the Feast of the Transfiguration. We find mention of the Christian blessing of fruit already in the early 3rd Century in THE APOSTOLIC TRADITION of St. Hippolytus of Rome. He offers no explanation as to why some things may be blessed but doesn’t allow certain things to be brought for a blessing, even though all food is to be received with thanksgiving.
Hippolytus doesn’t connect this blessing to a particular feast but writes:
Fruits indeed are blessed, this is grapes, the fig, the pomegranate, the olive, the pear, the apple, the mulberry, the peach, the cherry, the almond, the plum; but not the pumpkin or the melon, or cucumber or the onion, or garlic or any other vegetable.
But sometimes flowers also are offered. Let the rose and the lily be offered, but not others.
And for all things which are eaten they shall give thanks to God, eating them to His glory.” (pp 54-55)