I have a basic 1000x1000 Terrain inside a very basic scene (almost empty) but the rendering for long distance is providing like a blue tint or like some fog :
The Terrain settings :
I tried increase all LODs or distances I found, even on the Camera object.
How can I get rid of this ?
Check if you have deferred fog enabled in the Lighting settings, as it typically fades scenery far from the camera. There's also a setting for fog in other settings which may be the culprit.