The "Black Sun" is a Neo-Nazi symbol. Much like the swastika, it doesn't originate from the Nazi's themselves. the Sonnenrad (Black Sun) is an old Indo-European symbol.

Himmler renovated the Northern Tower of the Wewelsburg Castle (This Castle was supposed to become the Center of the SS), and on the floor is the Sonnenrad, this is a heavily modified version of the "Zierscheibe", a piece of jewelry from the European Iron age. They look fairly similar, but distinctly different, you likely wouldn't confuse the two. As far as I am aware, the meaning isn't certain. There are twelve spokes, we aren't sure exactly what Himmler meant by this, but some equate it to the 12 Knights of the Round Table. There are also 12 main gods in Norse mythology. Himmler and many others were big "fans" of Folklore and mythology, so it may be related to that. The Sun also commonly means "Victory" or "Unity", which is a major proponent to Nazi propaganda.

Here are a couple short reads about this topic:

From ADL

and LearnReligions