Outline of a selection

[prandus]

Here are several illustrative images:

- starting image: "flowers"
- foreground (what could be): "flowers_foreground"
- image obtained after global blurring then pasting of the foreground: "flowers1",
- image obtained after deleting the foreground, blurring, then pasting the foreground: "flowers2".

On "flowers1", the light colors of the foreground have smudged on the background,
On "flowers2", it's the transparency due to the erasure of the foreground that drools on the background.

All of these effects are completely normal; So the method I'm using isn't the right one...
So is there a better way to blur a background?
Thanks for your help;