What is a blackguard?
noun a rude or unscrupulous person who behaves in a dishonourable or contemptible way.
One of the characteristics of Hiberno (Irish) English is that there are many words commonly used in speech that are archaic in standard British English.
A good example is the word blackguard meaning a person "of low or worthless character." This seems to have originated in the disparaging references to the lower levels of the military and to the livery worn by bootblacks and other low status servants.
By the late Eighteenth century had become more directly associated with criminality. This association continued but blackguard had faded into obsolescence in British English by the 1950s. It has remained in common use in Ireland.