יורה דעה ר׳Yoreh De'ah 200

Se’if 1א׳

כְּשֶׁפּוֹשֶׁטֶת מַלְבּוּשֶׁיהָ, כְּשֶׁעוֹמֶדֶת בַּחֲלוּקָהּ, תְּבָרֵךְ: אֲשֶׁר קִדְּשָׁנוּ בְּמִצְוֹתָיו וְצִוָּנוּ עַל הַטְּבִילָה, וְתִפְשֹׁט חֲלוּקָהּ וְתִטְבֹּל; וְאִם לֹא בֵּרְכָה אָז, תְּבָרֵךְ לְאַחַר שֶׁתִּכָּנֵס עַד צַוָּארָהּ בַּמַּיִם; וְאִם הֵם צְלוּלִים, עוֹכַרְתָּן בְּרַגְלֶיהָ וּמְבָרֶכֶת. הַגָּה: וְיֵשׁ אוֹמְרִים שֶׁלֹּא תְּבָרֵךְ עַד אַחַר הַטְּבִילָה (טוּר בְּשֵׁם בַּעַל הֲלָכוֹת גְּדוֹלוֹת וְהוּא בה"ג דַּף פ"ה ע"ב וְרַשִׁ"י וְרָאָבָ"ן סִימָן שכ"ח וְש"ד), וְכֵן נוֹהֲגִים שֶׁלְּאַחַר הַטְּבִילָה, בְּעוֹדָהּ עוֹמֶדֶת תּוֹךְ הַמַּיִם, מְכַסֵּית עַצְמָהּ בְּבִגְדָהּ אוֹ בַּחֲלוּקָהּ, וּמְבָרֶכֶת.

When she takes off her garments, while she stands in her shift, she shall recite the blessing: who has sanctified us with his commandments and commanded us concerning the immersion; and she shall take off her shift and immerse. And if she did not recite the blessing then, she shall recite it after she has gone in up to her neck in the water; and if they are clear, she muddies them with her feet and recites the blessing. Hagah [Rema]: And there are those who say that she shall not recite the blessing until after the immersion (Tur in the name of the Baal Halachot Gedolot, and it is in the Behag, folio 85, column 2, and Rashi and the Raavan, siman 328, and Shaarei Dura); and so they are accustomed, that after the immersion, while she still stands within the water, she covers herself with her garment or with her shift and recites the blessing.