In Cycles you could create a material using an Add shader to add Transparent and Diffuse - so that you get the Diffuse effect from the plane added to the view through the plane as if it wasn't there. This could produce the following effect :

Here the 'blade' has an emission strength of 2 and there is a plane behind it (with the above material) between it and the 'background' cube and suzanne. The light bounces off the plane as well as the whole scene behind it being visible.

However, you would be better not going down that route as it's dependent on there only being 1 light source (the light saber) - otherwise the other sources of light would also appear on the plane (unless you hid them behind the plane or moved them to a different render layer). It's also bad practice since it's not physically accurate.

The other option would be to encase the 'blade' in a volumetric scatter material so that it scattered the light - but that would also cause complications as the volumetrics would scatter any light in the scene, not just that from the blade. Also it would be horrendously inefficient.

This is all far too complicated for such a simple effect in the compositor, using the Glare node set to Fog Glow such as that used here and here.

In Cycles you could create a material using an Add shader to add Transparent and Diffuse - so that you get the Diffuse effect from the plane added to the view through the plane as if it wasn't there. This could produce the following effect :

Here the 'blade' has an emission strength of 2 and there is a plane behind it (with the above material) between it and the 'background' cube and suzanne. The light bounces off the plane as well as the whole scene behind it being visible.

However, you would be better not going down that route as it's dependent on there only being 1 light source (the light saber) - otherwise the other sources of light would also appear on the plane (unless you hid them behind the plane or moved them to a different render layer). It's also bad practice since it's not physically accurate.

The other option would be to encase the 'blade' in a volumetric scatter material so that it scattered the light - but that would also cause complications as the volumetrics would scatter any light in the scene, not just that from the blade. Also it would be horrendously inefficient.

This is all far too complicated - much simpler to achieve this in the compositor using a Glare node set to Fog Glow. See here and here.