There are plenty of blackguards in British history.
Julius Caesar already uses the term Britannia in his Commentaries on the Gallic War, written in the 50s B.C.
Since I'm not sure how you meant this statement, I'll just say that blackguard is defined as a man who behaves in a dishonorable or contemptible way. I, of course, literally meant that the black peoples of sub-Saharan Africa had virtually nothing to do with British history. "British" in this instance to include England, Scotland, Wales and Ireland.