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.
A point which may be advanced in favour of OP's derivation is three allusions to the "black guard of the Dominic Fiars" 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: