I have used the SpecSer whatsapp service quite a lot; either directly with MAD or GRU, or with the centralized number (SCL? in Spanish / GRU in Portuguese?).
The service works very well, but I have mainly used it for seat assignment changes and upgrades - I do not know about more complicated issues. Their hours are quite flexible; early morning until around 23:00.
I primarily use the service for international flights (either to or from GRU), but on some occasions for domestic Brazilian flights.
I did not know about the Black Whatsapp service - but I will ask SpecSer about it at MAD before my flight the coming week.