Yeah the black switch is part of the switch to the right which means it is RCBO - sounds like it could be on any circuit that is connected on that side - it is probably an earth fault and you should switch off all the circuit breakers and try them one at a time until you find the circuit first - I would guess that its a circuit contected to something outside like security lights - its the most common fault I found because rain gets into circuits and cable deteriorates quickly that is exposed to the elements.
Get an electrician to do a test for you is the safest way.
Get an electrician to do a test for you is the safest way.