how to make black color transparent

I was making space background from one of the online tutorial , that guy told us to use any image with black transparent color, the idea is the image is totally black and white and by randomly selecting colors and applying them on black or white we generate new background for game.

I tried to apply the same technique to normal black and white pictures by using unlit transparent feature but instead of disappearing the black color it became more dark.
I tried to ask some people online about how to make a black color transparent but I did not get the answer.
Here is the link to the tutorial I’ve been following


Any help in this regard is appreciated.

You know this is really an image editing question try this

http://www.barryodonovan.com/2011/01/19/gimp-change-background-to-transparent

However why is that guy trying to create a Skybox from scratch! Just get a space skybox and use that.

https://unity3d.com/learn/tutorials/topics/graphics/using-skyboxes

There are even free ones like this

https://www.assetstore.unity3d.com/en/#!/content/25117