From blackguard -- either concerning street urchins who would blacken clients' shoes, or from the house servants who worked in the dark quarters of the house: a person whose conduct or character is disgraceful, disgusting, morally unfit; a thief with evil intent; a contemptible scoundrel; a term of abuse; a foulmouthed person

One who has a mania for music.

A person of dubious morals, sometimes with villainous intent and suspicious plots.
More often than not in cahoots with other shady characters.

A villain, a rogue, an evil or "black-hearted" person, hence abreviated low brow U.K. style to "blaggard"

A person who manages to get away with not doing something by way of talking their way out of it, or by just going on and on and on until someone else does it or you just give up and do it yourself or by the time they've finished it doesn't need doing any more.

from the British slang blag which means to steal or rob usually by force, a blagger is the robber who usually robs by force, the word blaggard is a variance on these two concepts a slang on the slang if you will meaning a rogue-ish bastardly person, a con.

A person that is mischevious and breakes the rules.