Because the author who coined the therm of first, second and third reich was german, so was the book, it never caught on with the angelos. It is called the black sun mainly for emphasizing its symbolic opposite of the golden, bright or yellow "real" or visible sun. And no they did not call it that back in the day, I think they just called it Sonnenrad but it hardly had significance as a word back then, that only came after the war.
who cares, whitey's cuck empire didn't last more than 8 years. now they're all getting TURKED
reich is just a cooler sounding word
same reason we call it the soviet union instead of the union of council socialist republics, it sounds foreign and spooky