Issue Description
If you are in 3D and you see a large flat blue grid, this is called the Editing Plane. It is an on-screen aid for working in 3D.
Solution
You can change the color/opacity of the plane if you are in a 3D view and go to View > Grid & Editing Plane Options > Grids & Background....
To toggle it off completely, go to View > Editing Plane Display.
• Read about: Editing Plane on our Help Center
• YouTube Playlist: https://www.youtube.com/playlist?list=PL2C6FA43216942F54