Blurring part of image?

How can I blur a part of a clip? I need to add a little box obscuring a part of the image but can't figure out how this is done within Kdenlive.

Effects list -> Blur and hide -> automask, obscure

Or else, make an alpha selection using rotoscope or alpha shapes, then composite with black or whatever

Thanks.
I get another issue which is that other effects is not affected by an pan&zoom effect. They are displayed in the original footage position.
This was the reason I didn't get automask and rotoscope to work.

If I create a rotoscope layer on top of pan&zoomed footage it is created and diplayed in the right position but when adding a box blur to this rotoscoped footage blur is displayed in the original footage position.
Tried to shift effect orders around but nothing seems to work.
What is going on!?

Well... Today things seems better. I removed and reapplied the composite transition/effect and now it works as it should. Strange. Same settings, new Composite.