Whose eyes were blind?
Rashi: Those who until now did not recognize this fear on them. This refers to Yisrael - "ha'Chershim Shema'u..." (42:18).
Radak: Yisrael in Galus were like blind, deaf, lame and mute.
Were they truly deaf?
Rashi: They did not hear the Nevi'im's voice until now.
Why will their eyes and ears open?
Rashi: I will give to them a proper spirit to fear me.
Malbim: Happiness is attained via intellect, speech and deed. Corresponding to intellect and investigation to understand true mindsets and Emunos, it says that their eyes will be opened to recognize these truths, either via perception in creations and wonders that they saw with his eyes, or via tradition from Nevi'im or their fathers - "Pen Yir'ah v'Einav u'Lvavo Yavin" (6:10). Their ears will be opened to hear the Nevi'im's words.
