I always heard that it was “kabbalistic” reasons. But that answer is always vague and who knows if its true. Also, I have heard the opposite, that the reason why Sephardim don’t have black lines is because (I think it’s) the Bais Yosef paskens that the tzitzis have to be the same color as the beged.
I actually like your first reason though. And even though today priests don’t do it, once we start a custom we don’t stop it even the reason is no longer present. Minhag avoseinu byadeinu.