Blue line on Garmin 3597 normal?

That is not a blue line as it is just like a horizon on the gps. Now if you ever do get a line that has to do with pixels of the screen contact garmin asap especially if still under warranty.

You have your Driving Map View set to 3-D and that is the horizon, the blue representing the sky. As I recall, it goes to dark gray at night. If you select North Up or Track Up, you get a straight down view and won't see the horizon.

Yup, it is a fake horizon, it will move up then disappear as you zoom in. When I first saw it in the 3-D view I did not like it but now after a couple of drives into the Smokies I find it fascinating to see the surrounding mountains reaching up into the "sky".

I used to hate that horizon line too on my 2798LMT but you get used to it and it's not so bad. The 3D view does help with orientation as opposed to an overhead flat view. I'm still not sure that the blue clouds are really necessary though...

Thank you everyone. That makes so much more sense now. I almost sent it back without asking, but glad I did. So far this unit has been great other than that "annoyance."

Had you sent it back for a replacement the new one would have behaved the same unless you chose a different model. It sure would have been an inconvenience to return so you were spared the aggravation by asking the question.
Enjoy your new purchase.

...and I have grown to love it. It makes me think I am looking at the sky.

It dissappears when you zoom in a couple of taps. Your Nuvi can be set to disable autozoom so if the blue line is a problem for you it can be set to a zoom level that will not move and you will not see the line any longer. The required zoom level would need to be set each time the Nuvi was started up however.