It was :
Not sure why when i lower the opacity the color never goes fully black
This annoying behavior occurs only when you are drawing with a very little opacity into an empty layer (it means the zone where the Alpha value is 0 or nearly 0).
In this case the blending equation can not pull enough opacity in the layer to reach the full opacity.
Moreover other drawing software seems to have the same behavior...
Mathematically it's possible to make an equation to have the desired behavior but the current restriction in the Blending equation provided by Direct X doesn't allow an easy implementation (perhaps it wil be possible with DirectX 12).
The "good news" is that this behavior doesn't occur on layer with full opacity
Here an example where I draw 2 beautiful blobs on the top layer and on the background.