After I filtered the water, there were some tiny black particles on the bottom of the filter, even though I poured it out and filtered it many times, it still didn't get rid of those tiny black particles, so what is it? Is it harmful to health?

The tiny black particles at the bottom of the bottle are just activated carbon from the BRITA^® filter. Activated carbon particles are made from coconut shells. Like all natural products, coconut shells are subject to natural variation in product quality. Sometimes this leads to abrasive parts of the coal entering your filtered water. It can help to remove the coal dust  by treating the filter element as described in the product manual. If customers drink this activated carbon dust, it will not affect their health because it is harmless.