It occurred to me to wonder when I ran across a French reference to practices of the Inquisition; it mentioned that imprisonment was fully intended to be torture that had a strong beginning when a guard-blacksmith shackled the prisoner to the wall of his/her cell. Could this functionary have been the original blackguard? Not in French, of course, where the word would be something like gardien-taillefer and the word itself is canaille.
I believe this derivation is quite impossible.
- The term "black guard" first emerges (according to OED 1) in the 1530s. Before the 1670s, every instance but one of "black guard" cited by OED 1 refers to a company, not an individual, "black" by virtue of their dress or their villainy or their dirtiness. (The one exception, upon examination of the source, refers to a guard who is, in fact, black—a negro.)
- In early use a guard watches over persons or places in order to defend them. The word is not used before the 19th century of an officer who watches over prisoners to prevent their escape—that is a warder or keeper or gaoler.
- It is unlikely that such an office as "guard-blacksmith" existed. A blacksmith in a fortress/prison or military unit would not be numbered among the common warders or soldiers; for instance, in 1414 William Merssh was capitalis faber at the Tower of London, a highly skilled craftsman, maintaining his own forge and directing his own operations, under the Clerk of the Works.
- If such an officer existed, it is inconceivable that he would have been called a black guard. A smith guard, perhaps, or an iron guard; but the smith is called a "blacksmith" only to distinguish him from a goldsmith or a silversmith, because he works in the black (dark) metal iron; in no other context does "black" refer to the smith's trade.
A point which may be advanced in favour of OP's derivation is three allusions to the "black guard of the Dominic Friars" in Foxe's Acts and Monuments, 1563— exactly when the phrase begins to be frequently encountered. One of these refers specifically to Dominic's preaching against the Albigenses and causing "many of them to be burnt; for the which he [...] was made patriarch of the black guard of the Dominic Friars."
But black here refers to the color of the Dominican habit, and guard is a collective, referring here and elsewhere in Foxe to the order's militancy. And for all Foxe's popularity over the following generations, nobody seems to pick the phrase up; Google Books finds no use of black guard from Foxe's day until 1700 which is not accommodated in OED 1's uncontroversial account of the phrase's development from "royal or military kitchen staff" to "camp followers" to "street urchins/linkboys" to "criminal low-life".
The origin of blackguard is a little obscure, but not so obscure as OED 1's admirable scholarly punctilio suggests, and not nearly obscure enough to accommodate this conjecture.
The OED has quite a detailed etymological note which begins:
The earliest uses apparently show simple phrasal uses, referring to a person or group of people (contextually or habitually) dressed in black or black in appearance, although many early uses also at least play on figurative meanings of black.