As far as I know the only way is to rebuild the shadow rendering code. You should light up pixels that are not covered by shadow, but now you make something opposite. Unfortunately, this is the way the shadows works in JME.
To be honest, drawing the shadows is a big part of the 3d rendering, if the built-in solution is not enough then try to look here: My next pathetic attempt with shadows - help - #74 by nehon
I’m using Perjin’s PrettyShadows (big thanks to him!) with a lot of modifications. It is not simple, it requires at least basic knowledge of shader’s programming, but it is possible to reach the effect you want in JME.
And thanks to the shadows I learned a lot about 3d rendering so it is worth to spend some time on it.
And to encourage you: Skullstone - a dungeon crawler game - #40 by FrozenShade here you can see what effect you can reach with that code 