This is just my theory ok perhaps someone already did this one or not. In the manga and anime it wasnt revealed why Tanjiro chose the black blade to all the blades that he couldve get.
But i think the main reason why blackblade is very unique its because the person who has it can potentially use all the breath techniques we saw him use waterbreathing technique combined with sun breath style that even the upper rank demon was amaze and shock to what happen. This is why i trully think that Tanjiro after the final battle he traveled the world to learn the other breaths the fire breath, wind, rock, etc etc to the outmost extent that when his decendants needs him one day they will remember tanjiro learning all this
But i think the main reason why blackblade is very unique its because the person who has it can potentially use all the breath techniques we saw him use waterbreathing technique combined with sun breath style that even the upper rank demon was amaze and shock to what happen. This is why i trully think that Tanjiro after the final battle he traveled the world to learn the other breaths the fire breath, wind, rock, etc etc to the outmost extent that when his decendants needs him one day they will remember tanjiro learning all this