I ran into this too on my site. I found the fix over at stackoverflow. Browse to the plugin area of wordpress go to slimbox and click edit. It takes you to an edit plug-in page. Then select the slimbox/stylesheets/jquery.slimbox.css file and do the following.
add z-index:1; to #lbImage { }
remove z-index:9999; from #lbCenter, #lbBottomContainer { }
and remove z-index:9998; from #lbOverlay { }
Credit goes to Lodder over on stackoverflow
https://stackoverflow.com/questions/15336690/slimbox-overlay-in-front-of-the-image