What does "mirror blue" mean?

I'm from Denmark and have heard the expression "mirror blue" in a song. I 
can't find it in any dictionary. Google gives 91.600 results when I search

for "mirror blue" (most of them refers to an album by Richard Thompson
from 
1994).

The song sounds:
"The ****d blue angel, who peers through the blinds,
Disappears in the gloom of the mirror blue night."

What is "the gloom of the mirror blue night"?

I would appreciate any help.


Jakob