The famous blur mask .
Alright it is really simple .
Open an image editor (or Construct 2 Build in image editor) and with a soft brush (no hardness) just click 2-3 times to make a soft edge circle like this (Be careful the background must be transparent and the color of the circle black)
Ok you are done with the first step now lets go to construct.
Open construct and insert a background image i choose something like this
Now insert your blury black circle as a sprite
And click add effects and in the blend sections
choose screen .Well done ... now you cant see the black circle. But wait if you again add an effect like ... lets say blur horizontal, you get something like this.
You can make a lot of variations with this technique .Transparent gradients etc
Dont forget to play with the opacity values and with the blur intensity.
That is it guys The famous Blur mask Cheers.