As far as I know, the dark saber does not have a canon explanation for its color. However, sabers themselves have very specific ways they gain color. In canon, sabers gain color based upon the emotion put into the blade. Essentially, the saber itself feeds off of your own energy and emotions, causing it to have a color that reflects whatever emotions you put into it. Red for example, is all the negative emotions you can out into the crystal. And white is all the positive emotions you put into the crystal, after a sith has already made them red. In legends, color was determined by your force alignment, depending on what side of the force you were closer to, your blade color would change. Sometimes, blade color was also determined souly by position. But the crystal itself held it's natural color.
Anyway, in legends, mandalorian culture considers the color black a color of justice. So, my theory is that the first mandalore to use the blade poured all of his justice related emotions into it, causing it to turn black.
However, I could just be making a connection where there wasn't one.
The crystal could just be naturally colored black. Black could signify a neutral alignment in the force, since black is a netural color.
The reason its black could be damn near anything. However, I like the mandalorian justice color connection. :)
