Have researchers found that your eyes are darker when you're constipated?

Probably not: I have never heard this, and i cannot think of any mechanism where constipation would cause darker eyes. Straining, of course, could be a cause of a broken blood vessel on the white of the eye. People with allergies often get a darkening around the eyes due to itching and rubbing, and people with allergies take antihistamines which can be constipating...Just a thought.