Implement fog (blur) on the map

Question

I will try to hold out for a few days, but I get no idea how I can solve this problem. There are some screenshots

when the first screen is displayed on the screen, than just displaying the user on the map and the other map is blurry, but when the user can switch to a different position than to clear each effect of blurring the position, like my second screenshot

please give me some logic how can i solve this.