Humm. Good question; perhaps do something like this, without the alpha re-evaluation loop...
Code:
set_motion_blur(100);
panel_motion_blur.transparent=0;
panel_visual_field.pos_x=20;
panel_visual_field.transparent=1;
panel_visual_field.alpha=50;
panel_visual_field.filter=1;
panel_visual_field.visible=1;

...if that doesn't blur things a bit, it may be that bitmap_motion_blur isn't being written(?)

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