For many times in the play we get a reference to blindness/vision. Why is that? What is the intent or purpose of these many references?

The repeated
references to sight / seeing / vision and blindness are a literary device called a
"leitmotif."  By examining a leitmotif in its different contexts, we can arrive a
theme of a literary work. Here it has to do with theof "sight" as knowledge,
especially knowledge of the self.  We "see" when we understand, just as we
"see" when we turn on the lights; we don't see in the dark.  In developing the
leitmotif of "seeing" metaphorically, as knowledge, and literally, as physical
vision,