What's the meaning of black color sample rate pass?

[fraggle]

Hello,
I trying hard to cut down rendering time without noise for an interior scene with lot of reflection (marble, stainless steel, silk paint...).
I put all the refl sub sample to 8 as a start to keep going with Vlado universal setting.
Then I look at the sample rate to find out what need to be crank up but I've got a lot of black area!
So what's the meaning of black color in the color sample pass?

Thanks

Oups, just find out that is is black on the VFB but red and blue when save as Jpg!!! Sounds like a VFB problem...

[svetlozar.draganov]

Make sure that all channels(R,G,B) are active:

Color Corrections are also taken into account.