This is very common and nothing to be concerned about.
The black particles are actually dust from the filter media but they are not harmful in any way.
This dust can be released from the filter cartridge for a few different reasons:
- The filter has not been pre-rinsed as per the manual instructions
- The filter has been pre-rinsed but may need further rinsing until the water is clear
- When topping up the product, the water is poured directly onto the filter cartridge and has flushed some of the particles out of the filter and into the product. It's best to avoid having the water land directly on the filter. Instead, gently pour to the side of the filter cartridge.
If you see dust in your product, simply rinse the filter until the water coming through is clear. Then rinse the bottom container and it will be fine to use again :)