Why do we cover our eyes when reciting the blessing on the Shabbat candles?

Normally, the blessing is recited before the performance of a mitzvah, but here we light the candle and only afterwards recite the blessing. This is because once the blessing is recited Shabbat has been ushered in, and lighting the candle would be forbidden. For this reason, we cover our eyes immediately after lighting to recite the blessing. Thus, our first enjoyment of the Shabbat light -- and the fulfillment of the mitzvah -- occurs after the blessing.