It is important to identify the way in which blindness in this brilliant tragedy symbolises far more than the ability to be able to use your eyes. It also points towards the inability to see things the way they are, particularly when it comes to understanding yourself and the motivations of characters around you.
Bearing this in mind, if we consider the blindness of Gloucester, we can see that this stands as a symbol for the metaphorical blindness that both Gloucester and Lear suffer from. The blindness therefore functions as something that reinforces the similarities between these two characters. Both, after all, have children that are loyal and devoted to them and children that are definitely not, and both, tragically, show blindness in their inability to discern which children are loyal and which are not, and demonstrate this blindness by banishing the loyal children and choosing the disloyal ones to be their heirs.
There is tremendous irony in this play that it is only after Gloucester has been blinded that Lear has descended into lunacy, two things that should prevent clear vision and insight, that these characters begin to truly "see" the massive mistakes that they have made. Note the way that these two characters meet in Act IV and talk about their blindness and how their mistakes have resulted in such tragedy.