It’s sad that this contemptuous term for a scoundrel, a man who behaves in a dishonourable or contemptible way, has fallen out of use, since it carries a big punch. Our usual pronunciation as “blaggard” obscures its curious composition. Who or what was the black guard that got itself such a dreadful reputation? If I had a time machine handy, I’d go back to about 1500 and ask some pointed questions of Londoners. Failing this device, matters have to remain somewhat obscure.
The earliest recorded use, by a few years, was in 1535. Then it referred to low menials in a royal or noble household. They were the ones who looked after the pots and pans and other kitchen utensils: the scullions or kitchen-knaves. Nobody knows for sure why they were said to be black — perhaps the colour of the pots literally or figuratively rubbed off on them. A slightly later sense is of the rabble that followed an army about: the servants, camp-followers and general hangers-on (here black presumably has its common derogatory sense). There seems to be a third sense, which refers to a guard of attendants or soldiers who were dressed in black; it’s possible that there really was a Black Guard — so called — at Westminster about this time (there are account records that refer to them, but nobody has any idea who they actually were).
By the eighteenth century, the term was applied to children and young people who made a living any way they could, either as boot blacks or general assistants to soldiers (presumably this was a joke on the literal form of the word). Our modern sense appeared about 1730, and was a highly offensive term for a scoundrel or villain, or any low worthless minor criminal.