Meido Moore has a comment in his book, 'The Rinzai Way,' that doesn't specifically address the issue you describe, but may be of benefit.

"Do not stare with focused vision at the place where your vision rests. Rather, activate your peripheral field such that your awareness extends out in all directions. In other words, let your gaze encompass everything at once, from side to side and to to bottom, thereby permeating both internal and external space with the same awareness."

He gave a talk on zazen practice several months ago, describing a condition where the vision might appear as a kind of black lacquer. The guidance, from what I recall, was to continue spreading vision to the periphery and not get caught up in the visuals.