Background always black on adding skybox

I made a material and gave it a red colour. After that I added it as the skybox of my Main Camera. But the camera is showing bluish-black colour. Why is this happening? I tried re-installing Unity but my problem was still not fixed.

As you can see in the image, my material is red, my camera's background is also red, but the preview is showing black colour, except when I take a default material.