This is the final result want to obtain (obtained using Blur effect in my video editor on that mask)

No way to use gradient tool to better control the transition size?

Ok that works!! I'm dying for such a stupid task, really angered with my inability to obtain these trivial things. Maybe using Inkscape for such stupid task is such as using a cannon for a bug, but Paint or Paint.net didn't achieve the result in an easier way. So next task is: did want the border of...

Thanks for your reply.

I think the problem is the export. Every time I export to PNG the central rectangle became a white and not transparent.

Need I to care about something exporting into png?

Need to create a mask to be used as overlay with my video editor program (PowerDirector). Using Inkscape 0.91 Need to be a 1920x1080 (FullHD) black rectangle with a transparent hole in the center, sized 728x540. I did define the Document properties with FullHD size, background white w 100% transpare...