Blur affects to all the layers that are under it. You cannot do selective blur at the moment. It is a feature request. You can do a partial workaround for that doing this:
Encapsulate the stuff to partially blur into a paste canvas and export it. Call it "background" for example.
Do the following composition:
Encapsulate (Composite)
-Encapsulate (Onto)
--Blur layer
--Paste Canvas [background]
-Blur region mask
Paste Canvas[background]
In this way you composite a cropped blurred background over it self giving the impression of a partial blur. It is a fake, but it is the only way I know for the moment.

This is done with a feathered inverted circle as blur region mask.
-G
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