Have you ever wondered about this item and how it became popular? (Don't say US despatch riders and bomber jackets as they were brown). I know a lot of people make the mistake in thinking it was something to do with the SS but it's likely that it goes back to the anarchists in the Spanish Civil War. This page on sourcing a correct style of coat by UK SCW reenactment group La Columna has some interesting notes and photos. They quote Carl Marzini, an American volunteer in the Anarchist Durruti column in late 1936, who states in his memoir, " Of course you could tell an anarchist anywhere, because all anarchists dressed the same way. They all had leather jackets, or at least if they could get them, that's what they would wear 3"
Of course before that the Cheka - Bolshevik police - wore black leather - possibly as leather coats were then a status symbol in Russia or because they could easily be wiped clean. These people fought Anarchists in the Russian Civil War who were at the time notable because of their long hair but maybe they took the idea of black leather from the Cheka and this went then onto the Spanish Civil War. Any ideas out there costume researchers?