Minimap not showing background

I’ve created a small minimap and masked it so it becomes round using a 2d sprite texture. It’s fed from a camera render texture. In the camera preview, I can clearly see the background which I set to a solid blue colour, but in the minimap mask, it shows white instead. How can I correct this? Thanks.

Click on the blueish color picker by Background and increase “A” (alpha) to 255

Detailed Unity3D Minimap Tutorial here

Or watch it on Youtube