The usual scheme is to have two copies of your clip (one normal and one blurred etc) separated by an 'alpha mask':

Track 1 normal clip

Track 2 the mask eg a white circle on a black background .jpg image with 'alpha' chroma key applied.

Track 3 clip with effect applied.

An alpha mask lets you see the track above where it is white and the track below it where it is black (or visa versa). Alwyn, who used to quite active on the Magix form some time ago, has a tutorial on this called masking a moving face. http://www.aaproductions.net/mep/tips/tutes.htm

A search of this site will no doubt find additional help.

Regards,

Carl