OK. I exported the clip as an MP4 1080p file, and it was much lighter. This was with the Levels adjustment. So turned off the Levels adjustment and exported it again, and it came out spot on. It is just looking darker in the timeline.
The still image has four matted video layers above totalling about 25% of the frame. The video layers are all 1080p 25 MXF files from an XF300.
BTW My >1500 posts to the Adobe forums are nearly all from the Photoshop forum. I am still learning Premiere Pro, but enjoying video much more than stills nowadays, so trying to catch up.
>still learning Premiere Pro
Some links to help... ignore any you've already visited
Right side of http://forums.adobe.com/community/premiere
A "crash course" http://forums.adobe.com/thread/761834
A Video Primer for Premiere http://forums.adobe.com/thread/498251
Premiere Tutorial http://forums.adobe.com/thread/424009
-and more from Kevin http://forums.adobe.com/message/4714153
Premiere Pro Wiki http://premierepro.wikia.com/wiki/Main_Page
Tutorial HD to SD w/CS4 http://bellunevideo.com/tutorials/CS4_HD2SD/CS4_HD2SD.html
Exporting to DVD http://help.adobe.com/en_US/premierepro/cs/using/WS3E252E59-6BE5-4668
Color correction http://forums.adobe.com/thread/892861
After Effects Tutorials http://www.videocopilot.net/
Surround Sound http://forums.adobe.com/thread/517372
Photo Scaling for Video http://forums.adobe.com/thread/450798
-Too Large May = Crash http://forums.adobe.com/thread/879967
-And another crash report http://forums.adobe.com/thread/973935
Video Scaling https://blogs.adobe.com/premiereprotraining/2010/10/scaling-in-premiere-
Encore Tutorial http://www.precomposed.com/blog/2009/05/encore-tutorial/
And more Encore http://library.creativecow.net/articles/devis_andrew/
Regions and NTSC vs PAL http://forums.adobe.com/thread/951042
-and Regions http://forums.adobe.com/thread/895223
You can't necessarily trust a computer monitor combined with software to show you an accurate image, especially where video is concerned. You should use a properly calibrated external TV for this type of quality control.