You have to render the frame as an animation to get the blur.
You only need a 2 frame anim to get motion blur, set the scene in frame one, then in frame 2 move the object you want blur on. Goto movie settings, enable 2d motion blur (or 3D motion blur in the main render settings), set Format as image files, set the renderer to the render option (eg, firefly, P4), then set the 2nd frame to be rendered in the time Span option (Start 2, End 2). I used the 2 frame anim technique in this pic that I did to get the background blurred, http://www.renderosity.com/mod/gallery/index.php?image_id=582306 . 

For postwork blur I would render the background, save the image. Start a new scene with the object (eg, spaceship) to be blurred, then render it with no background. Save as a .tif or .png to get the alpha channel. In your graphics program load your bg image, then load the other render, use the alpha channel to get a crisp selection of the spaceship and copy/paste onto a new layer over the background image. Then use a motion blur filter on the spaceship to get the required amount of blur.

Hoper that helps.