A website goes dark is user clicks anywhere on the web page. Only affects Internet Explorer 9 users (IE 10 maybe affected as well, but it is still in beta haven't been tested).
Website used Lightbox 2 to display images (v2.04). It seems that this version is not compatible with IE9. The dimming effect is supposed to darken the website only when a pop-up lighbox image is displayed, however, in IE 9 it was dimming whole website after clicking anywhere on any page, even without images at all.
Update Lightbox 2 to the latest version. At the time it was version 2.51, which resolved the issue.