I have consulted the entry ‘blackguard’ from the Big List and now would like to know how long ago ‘blackguard’ became a verb of good standing. I imagine how it became a verb is obvious--the villanous type that ‘blackguard’ describes would often employ vituperation.

From the OED: first citation

2. trans. To treat as a blackguard; to abuse or revile in scurrilous terms.
1823 COBBETT Weekly Reg. XLVIII. 642/2 You, in your quality of Saint, may claim a right to becall and to blackguard, as much as you please, any portion of the rest of mankind.

It should be noted that the ordinary pronunciation is something like “blaggard”. I may have heard people saying “blaggard” without knowing what it meant, but it wasn’t until watching Eugene O’Neill’s “The Ice Man Cometh” (well into my twenties, I think starring the incomparable Lee Marvin) that I ever knew how it was pronounced, since I had that infrequent experience of actually having read something I was watching. It is virtually unknown Out West here other than in print I would say.