A bride without a blessing is forbidden to her husband like a menstruant who has not immersed. And from where do we derive the blessing of bridegrooms from the Torah? As it is said, "And they blessed Rebecca." And from where that even a widow requires a blessing? As it is said, "And Boaz took ten men" (Ruth 4:2).
The Wedding Blessing Drawn From the Torah
Yalkut Shimoni on Torah 109:10
כַּלָּה בְּלֹא בְּרָכָה אֲסוּרָה לְבַעֲלָהּ כְּנִדָּה שֶׁלֹּא טָבְלָה. וּמִנַּיִן לְבִרְכַּת חֲתָנִים מִן הַתּוֹרָה, שֶׁנֶּאֱמַר, וַיְבָרְכוּ אֶת רִבְקָה. וּמִנַּיִן שֶׁאֲפִלּוּ אַלְמָנָה צְרִיכָה בְּרָכָה, שֶׁנֶּאֱמַר "וַיִּקַּח בֹּעַז עֲשָׂרָה אֲנָשִׁים".