A negative light level to be used like:

 

 

public BlockNegativeLight()

    {

        super();

        this.setBlockName("negativeLight");

        this.setNLightLevel(1.0F);

    }

 

 

 

The cauculation would be done by following one of those examples using a "Negative Torch" or both but for different "negative" light strenght

 

 

How exactly do you expect something like that to be implemented?

Lighting is 4 bits, thats all, in order to expand that to allow for native numbers we would have to break the chunk format.

Not to mention propagation is a bit.. iffly. Trying to figure out where light should be 'negative' and where its just dark...

This is not something that will go into Forge. It's a major breaking change in the engine.