so, if you want to use a mask, you ll have to use a postprocess blur. and mask it (that s what is used in your links) not camera blur.
you can find blurs i made here : FREE custom nodes non kernel blurs - Community Content, Tools and Tutorials - Unreal Engine Forums
but you may have to modify them a bit if you want a lens blur look (be aware that it will have a pretty heavy performance cost)
you may need this : Post Processing Content Examples | Unreal Engine Documentation
it s a tutorial for postprocess, based on the content exemple project for UE.
hope it helps !