Is this the effect you are looking for? (See the attached screenshot)
If Yes, then it’s simple.
Add an Image as background for the area required to be blurred in the UMG. Use a normal image (you can chose any color depending on other elements of UMG). Now, play with the Alpha value of the image to give a blur effect. With higher value blur will also increase.
If you want to add more effects make a png image with different filters in Photoshop and use that as background image. Play with Alpha as above.