Thanks, the feather helps me!
Meanwhile I've found the way to achieve my goal:
1) create a layer select with the brush the foreground that shall stay untouched, create mask on this selection, hide painting layer
2) invert mask and copy selection
3) create a new layer and copy the selection to it (now this layer has just the background, the foreground is cut from it)
4) blur this new layer and copy it a couple of times
5) on each of the blured layers create a transparency gradient from the corner (non-transparent) to the position of the foreground (100% transparent)
6) now activate the background layer and all the blurred gradient layers
7) voila: now foreground is still untouched and sharp, and the set of gradient blurred layers provide the softened background.
8) in case the foreground seems to be too sharp compared to the background use the feather tool on the mask created in step 1)...
thanks a lot!