Not able to take screenshots of the Xfinity Stream website

I just posted in the on demand forum regarding a problem with a show that has distorted video.  I was hoping to take some screenshots of the problem and post them in that discussion to illustrate my point, but there seems to be something blocking the ability to take screenshots of the site!  Any attempt to take a screenshot results in a big black rectangle.  It would be helpful if we were able to take screenshots in order to show you what's going on when we have problems.