I haven't tried flat-blue, but I have seen this on Nevato as well. The problem with Nevato was that it relies on a number of hard coded aspect ratios that your TV/monitor has to match exactly. The aspect ratio for 16x9 is 1920/1080=1.777778 but there is a special case for 1366/768=1.77865. My TV was 1360/768=1.77083 which didn't match exactly, so it failed to recognize it as a 16x9 screen.
I guess that the fix is to rewrite Nevato to use if/else statements instead of a switch block when translating the aspect_name so that ranges can be used instead of exact values. (But I haven't bothered to try, and I haven't written squirrel before)