re: Blue Tail Fly
What ever they are, their range seems to be quite large.
From here in middle of Ontario Canada to Tampa Florida is a fair distance.
There are some similarities in the areas where the observations were made.
The outhouse was in an area that is highly wooded, the trees offer effective shade, there is a river nearby, the closest town is about 5 miles away.
The picture on this site has some resemblance to the one I saw, the difference is that only the rear section was blue, the front section was black.