[jow to make Blurry Stock]

how to make a blurry strock ... i do not know if that what it is but i want to know how to make my stock look like the second picture  

 

 

as you see obviously  it looks sharp 

 

 

i want it look like this, i have tried gaussian blur but it does not give me the outcome i want. ..

[opposite of subtract ?]

 

I'm new on Affinity actually new on designing. and im trying to learn how to design by emulation so. there was this video on Youtube and I'm trying to copy what he is doing ... anyway I faced this issue

 

i dont know how to cut the part of the rectangle that lay outside the shape?