I know that having a black colored blade symbolizes unknown element but why did he got black colored blade instead of blue even though he has a known Water Breathing Technique?
Color of Nichirin blade, till now, is shown to have not much significance apart from manifesting differently based on the owners. Urokodaki was surprised as Black is a rare color and there's been few users with it, hence the saying emerged that those with black blade won't go far.
Haganezuka implied that family lineage also impacts the color of blade, here's what's been revealed in the manga which could explain the black color:
Tanjiro's ancestors Sumiyoshi and Suyako were acquainted with the Breath of the Sun user, also it's been implied that the Dance of Fire God passed down in Kamado family has some relation with the orignal breath the Breath of Sun.
In chapter 113 in Tanjiro's inherited memories he sees Suyako talking to the Sun Breath user about his sword, mentioning that a Samurai's sword changes from Obsidian Black to Red only when he fights. Tanjiro assumed that the Breath of Sun user used a technique to change the color of his sword, Tanjiro obtained the same effects thanks to nezuko's blood, his sword changed color to a burning red and he gained explosive battle strength. The black color may be due to his lineage or due to learning the dance of fire god (if learning breaths have any impact on it)