It is a minhag. Period. Moshe Rabbeinu did not own a Borsalino, I am quite sure. Neither did Rambam (he wore some type of turban).
The black hat is part of today’s Yeshivish levush. One can be an extraordinary, frum person without wearing a black hat. One can likewise be a waste of space int eh Yeshivah, even while wearing a hat. it is what is UNDER that hat that makes all the difference. If you want a boy with a black hat, only go out with such boys (but make sure their middos match their appearance). If you don’t care about the outward appearance, then go out with guys who do not wear black hats (but make sure their middos, likewise are worthy of you).