If the filters have not been cleaned and primed properly or if your tanks have not been thoroughly cleaned and rinsed, the water from your first filtrations may have a peculiar, even mineral taste. You can empty the first 2 or 3 filtrations.
If this persists, make sure that the black seals of the Black Berkey filters are well positioned on the upper tank side and not above the butterfly nuts on the lower tank side.
If necessary, proceed again to a more rigorous cleaning and priming of your system.