So what is the significance of the edge here (or in Newman or anywhere)?
On the one hand one could say – with all those who are sceptical of or disdain modern art – that there is none, period.
On the other hand, one could develop a theory about the significance of the edge and this one in particular, finding irrefutable reasons why it has to be exactly the color it is, why exactly it is in the position it is, what catastrophical things might happen to the painting or its originator or all of mankind if the edge were moved by a millimeter or a few pixels, etc.
One could reference the teachings of Mondrian, Kandinsky, Rudolf Steiner or whoever else and turn it into a Philosphy of the Edge. Possibly write a book about it. I actually already have a title in mind: Ooh Aah and Gaga / A Philosphy of the Deep Edge.