According to the gemara, as explained in the kitzur, 142:6
חַיָב אֱנָשׁ לְבַסּוּמֵי בַּפּוּרְיָא עַד דְּלָא יֵדַע בֵּין אָרוּר הָמָן לְבָרוּךְ מָרְדְּכָי. וְלַפָּחוֹת יִשְׁתֶּה יוֹתֵר מֵהֶרְגֵּלוֹ
This strange combination of Hebrew and Aramaic (Puria for Purim, Sumey for Suma) leads to the following translation:
"A man is obligated to become blind on Purim until he doesn't know [the difference] between 'cursed is Haman' and 'blessed is Mordechai'. At least he must drink more than his legs (others say "more than he does on his holiday")"
Why would not being able to see lead to not knowing the difference between two characters in the megillah?