I remember my hate for the Blue Line!  You cannot make it go away if you have any kind of Earth atmosphere.  Only space scenes or solid colors will not have a line. 

One thing to keep in mind about Bryce and Poser.  Bryce is primarily a landscape program.  Every landscape in the world has a horizon.  If you look at any other landscape program they will have some kind of horizon.

Poser is primarily a figure modelling program.  It does not need a horizon.  Poser can be used to make full scenes with horizons but that is not it's focus.

There is an easy way to avoid the line and other problems related to pointing the camera at the sky. 

Make or find a high resolution picture of a sky you want as a background.
Delete the ground plane.
Create a 2d Face.
Go to the texture editor by clicking 'M' next to the 2d Face.
Change the amdient to 0.
Click the first empty circle next to 'diffuse'.
A texture will come up.  Click the 'P'.  Then the very light pink dot at the top left.
You should see 3 big boxes at the top of the screen.  Click 'Load' on the 3rd one.  
Put the sky picture on it.  Then click the check mark.  
Click on the small grey triangle to the left of the texture boxes.  It is not in a circle.  It says 'Material Options'.  Turn off the 3 shadow settings by clicking them. 
Click the check mark.
Turn the camera to look straight at the 2d Face.  
Increase the size of the 2d Face and move it away from the camera. 
Move the sun to be behind the camera or add some lights to make the picture brighter.  

this is an easy way to make backgrounds for scenes.  There are some limitations.
All actions must be in front of the 2d Face.  The original size of the sky picture is important.  Bigger is better.  At leat 512x512.  1024x1024 is best.  You can find sky pictures here in R'osity 
Free stuff, Google Images, and many other places.  

Sorry for the long description.  Hope it helps.  Let me know if it works for you.  Try not to get too frustrated.