Once coming back from Salvador we encountered a line squall in approximately the same area. As we got closer i.e. about 80 - 160 nm we could see a dark spot on the radar i.e. a clear area of sky and that is where we went. However all hell broke loose as we entered what must have been a dying cell fortunately we where only in it long enough for me to get the engine and airframe ant-icing and ignition on before coming out the other side.
There was a BA behind us who I warned but was to late as they had also entered it as did TAP behind them. We spoke to them afterwards and they also had no returns on their radar
We were lucky in that it was very short lived and that the cell was in its decaying stages. As you can imagine there was a long discussion about what had happened between the three of us that where on the flight deck that night. The conclusion was that there was nothing we could have done to avoid it as there were no returns on radar and it was pitch black night so no chance of a visual sighting.
With regards to use of radar when I am CM1 I generally keep my radar on 80 with .5 down tilt with CM2 on the next range up. If I start getting returns I go down a scale when the returns are half way down my screen. With the CM2 also coming down that way i get the strongest returns for avoidance whilst using the CM2 to pick a route around what might be behind.