It is customary that after the Shabbat Candles are lit, both hands are waved towards the face (symbolically drawing in the light of the candles and the sanctity of Shabbat) and the eyes are covered. The blessing is recited with the eyes still covered. Why?
The sages taught that a blessing should always precede the action, meaning that the blessing for a mitzvah is recited before performing the mitzvah. For instance, one says a blessing over an apple and only then eats the apple; one says the blessing over the Chanukah candles and only then lights the candles.
The laws of Shabbat, however, prohibit the creation of a flame. Since Shabbat is accepted as soon as the blessing is recited, one must light the candles (do the action) before making the blessing. By covering the eyes before reciting the blessing, one is unable to benefit from the light of the candles until after the blessing is said. When the person making the blessing uncovers her/his eyes, it is as if she/he is seeing the light of the Shabbat candles for the very first time. In this way, the blessing has been recited before the action of the mitzvah is performed.
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