Long Black Veil is such a sad song. The sadness does not come from the obvious imagery of death, such as the grave or scaffold, or the mourning woman in the veil and the night wind.
Rather, there's a deeper sadness about sin. It bears down on the listener. The song touches upon this original condition within us, in its ontological reality: the universal condition of us all. The song grieves with this heavy weight - not laying it on with garbled and overwrought emotions - but it does so in simple terms, the meanings resolving in their own good time, and with an almost upbeat rhythm, which only goes into making the song that much more deep-reaching.
It is about how sin closes in, and closes us in; it is about the awful claustrophobia of sin. This sense is most imparted by the refrain: "Nobody knows, nobody sees; nobody knows, but me".
The narrator is referring not to some special knowledge that makes him higher than others, but to something of which he is ashamed and cannot admit, so he hides it, even if it means that hiding it will get him executed for another's sin of murder of which he (the narrator) is innocent.
There is something inexpressible in that the narrator's sin comes to haunt and condemn him through another's completely unrelated sin and that the two sins are very different; that it is not necessarily the external perversity of sins that make one ashamed, as much as a deep knowing within the soul that it has commited an offence that is contrary to its ingrained dignity.
On a more lofty level there is perhaps a notion in the song that before the Eternal Judge not one of us has an alibi.
The song wasn't originally written by Johnny Cash, but clearly Cash took the song and made it his own. I haven't heard covers of the song by other musicians, and don't really care to. The song was written for Johnny's voice.