I would like to implement a fast smooth blur shader using CG shader language in unity.
What is the best and standard way to implement it on mobile devices?
The king of thieves game has used it
You don't need to make blur shader yourself. Unity already made blur and other image effects and made it part of their Standard Assets. You can get it here.
There is a new image effect for Unity called "Post Processing Stack" which is faster than the one I linked above and you can get that here. I suggest you use this version.