How to paint Weight Black to Weight Blue?

Hello there!
My question here is how can I get the part in black to blue. (see the images)
I don’t know exactly what the weight black does but I think that it disturbing the natural weight paint.
When I paint with any value or even 0.05, the Vertex works like was painted with 1. And I think is because that are the black color on.
The Weight Tool “Clean” expands the black, and paint with Brush:Mix with Blend:Substract (and other combinations) turns black also.
Paint in the parts blue gets the good results with varied values. Diferent in the black 0.0/1.0

For any more clarification about it, ask me please.