Why is the divine light that leads to divine union called by the soul the Dark Night? 

The clearer and more manifest are divine things in themselves, the darker and more hidden are they to the soul naturally; just as, the clearer is the light, the more it blinds and darkens the pupil of the owl.  The more directly we look at the sun, the greater is the darkness which it causes in our visual faculty, overcoming and overwhelming it through its own weakness. 

Mystical theologians call this infused contemplation…A Ray of Darkness!

This is the exact image that John has of the Ascended Messiah in the Revelation…His countenance was like the sun shining in its strength. And when I saw Him, I fell at His feet as dead.

Once I understood the darkness is caused by the great light of his presence at work in the cavern of my heart then I was given exceeding hope for a joy that is and would be everlasting in my heart.

If the light in me is darkness, how great is the darkness.  The light of Christ comes to shine in a dark place.