The Black Sun also relates to the Prima Materia in Alchemy.
Behind many of the Gods of Death - there is also a black sun. The reason for this is that it represented the Sun rising in the Otherworld. Although it also has its' classic germanic roots which connect it to the Spiritual Sun or the Source (who the germanic pagan tribes viewed as Wotan... who is different from the Norse Odin in that Wotan is also Dark and practices homosexual practices with Loki, as well as other shamanic practices).

Himmler took the symbol as a means of trying to recreate a germanic religion, which all he ended up unearthing was the exoteric - innargard herd religion.