(Genesis 27:1) "And it came to pass, when Isaac was old, that his eyes were dim." Rabbi Isaac opened: (Isaiah 5:23) "Those who justify the wicked for a bribe, and take away the righteousness of the righteous from him." Anyone who takes a bribe in order to justify the wicked, this is Isaac: because he justified the wicked [Esau, accepting the savory dishes Esau brought him as a kind of bribe], his eyes grew dim, [as it is written] "And it came to pass, when Isaac was old, that his eyes were dim," and so forth. (Proverbs 17:15) "He who justifies the wicked and he who condemns the righteous, both alike are an abomination to the LORD." It was not because Rebecca loved Jacob more than Esau that she did this thing [arranging for Jacob to receive the blessing]; rather she said: "Let him not enter and lead that old man astray," on account of "both alike are an abomination to the LORD." And because he [Isaac] justified the wicked, his eyes grew dim.
Isaac Went Blind for Justifying the Wicked Esau
Yalkut Shimoni on Torah 113:5
(בראשית כז א) וַיְהִי כִּי זָקֵן יִצְחָק וַתִּכְהֶיןָ עֵינָיו, רַבִּי יִצְחָק פָּתַח (ישעיה ה, כג) "מַצְדִּיקֵי רָשָׁע עֵקֶב שֹׁחַד וְצִדְקַת צַדִּיקִים יָסִירוּ מִמֶּנּוּ", כָּל מִי שֶׁנּוֹטֵל שֹׁחַד בִּשְׁבִיל לְהַצְדִּיק אֶת הָרָשָׁע זֶה יִצְחָק עַל יְדֵי שֶׁהִצְדִּיק אֶת הָרָשָׁע כָּהוּ עֵינָיו וַיְהִי כִּי זָקֵן יִצְחָק וַתִּכְהֶיןָ עֵינָיו וְגוֹ'. מַצְדִּיק רָשָׁע וּמַרְשִׁיעַ צַדִּיק תּוֹעֲבַת ה' גַּם שְׁנֵיהֶם, לֹא מִמָּה שֶׁהָיְתָה רִבְקָה אוֹהֶבֶת לְיַעֲקֹב יוֹתֵר מֵעֵשָׂו עָשְׂתָה אֶת הַדָּבָר הַזֶּה, אֶלָּא אָמְרָה לֹא יֵעוּל וְיִטְעֵי לְהַהוּא סַבָּא עַל שֵׁם "תּוֹעֲבַת ה' גַּם שְׁנֵיהֶם" וְעַל יְדֵי שֶׁהִצְדִּיק אֶת הָרָשָׁע כָּהוּ עֵינָיו.