The physical blinding of Gloucester is perhaps one of the most horrific scenes in all of Shakespeare. It serves to reinforce his blindness to his world. He cannot see the truth which is that his bastard son is deceiving him. Too readily he believes the lies. Perhaps it is his guilt at his treatment of Edmund. This just goes to show that he really doesn't know his sons.
Lear also does not know his children. If he did, he would not ask the question he does ask---how much do you love me? Goneril and Regan play daddy's game. They both know that he wants to be flattered, so they give the old man what he wants. Cordelia, on the other hand, does not understand this and "cannot heave her heart into her month". In a sense, she is blind to what her father wants and perhaps at this stage of his life needs. Instead she is honest. Here, they are both blind in their own way. Lear's journey through madness opens his eyes to the truth but it is too late to save either of them.
Both Lear and Gloucester are blinded by their egos and pride at the beginning of the play and must discover enlightenment only through the pain they suffer.
In King Lear, the theme of sight and insight is developed through the motif of blindness. King Lear, Gloucester, and Edgar are blind for much of the beginning parts of the play; their blindness means that they do not have the insight to see evil working around them. King Lear is taken by the praise of his daughters Regan and Goneril, and he does not see that they are simply trying to manipulate him to get his possessions. Similarly, Gloucester is blinded by his son Edmund and believes that Edgar is out to murder him. He does not consider that Edmund is exacting revenge on him. Finally, Edgar is hopelessly blind and believes that all will work out in the end. As the play continues, these characters fall into more physical realms of blindness, but with this comes mental insight. Lear in his madness finally understands that Cordelia is the daughter who truly loves him, and Gloucester and Edgar recognize Edmund's schemes. So the mental and the physical are at odds and work to develop the theme of sight and insight in the play.