This was the only Makkah where Paroh said: 'ההַ צַ דִ יק Hashem is right! Why? Chazal say: אֱ מוּנַ ה זֶה סֵ דֶ ר זְ רָ עִ ים (agriculture involves Emuna). A farmer has Emuna because he always looks upwards to Shamayim for rain, and he thinks of Hashem. But in Mitzrayim there is no rain (instead, the Nile River irrigates the fields). A Mitzri looks downwards, and indeed he is heavily involved in: צִ רְ אַ יוּת earthliness, and he becomes: שְׂ טוּפֵ י זִ ימָ ה steeped in immorality.
We can suggest that since Makkas Barad was an Onesh (punishment) directly from Shamayim, therefore Paroh connected to Hashem and he did Teshuva.
And notice that by all the Makkos it says: וַיֶעְ תַּ ר אֶ ל ה' ,וַיִ צְ עַ ק משֶ ה אֶ ל ה' Moshe davened that the Makkah should disappear. Only by Barad it says: וַיִ פְ רֹּש כַפָ יו אֶ ל ה' ט לג he stretched up his hands to Hashem, which appears that Moshe is raising his hands and aiming toward Shamayim.
