Samba is a loss of motor control. It is a partial loss of physical or mental integrity and generally occurs up to 15 seconds after reaching the surface, normally during your first breath after a dive. It happens due to not having enough oxygen in your brain.
A Blackout in freediving is often called Shallow Water Blackout (SWB) as it most often occurs while ascending the last few metres of water. It happens due to lack of oxygen due to rapid changes in pressure.
Very briefly, it is thought that Hyperventilation is the leading cause of blackout as it lowers the amount of CO2 in your body which lessens the urge to breathe. The Shallow Water Blackout Wiki explains this all in much more detail: http://en.wikipedia.org/wiki/Shallow_water_blackout