Nowadays the word blackguard is a somewhat archaic insult, but the black guard were originally the kitchen servants, who were so-called because they had to deal with coal. As often, words for lower-class people become words for bad people: villain used to mean 'serf' (this meaning is usually spelled villein these days, but the words are the same), and before that it meant 'village-dweller'.
The narrator of Chaucer's Canterbury Tales says of the Knight:
He nevere yet no vileynye ne sayde
In al his lyf unto no maner wight.
which (in addition to being a spectacular example of Middle English multiple negation) means that the Knight was 1) never rude and 2) never behaved like a peasant.
The standard pronunciation of "blackguard" is "blaggard". It's typical for compounds to be pronounced as single words when they get established, and later to undergo sound-change as if they were single words. For example, English has created three separate compounds from house and wife: the modern formation housewife, the Middle English hussif (obsolete now, but still current in the 19th century) meaning 'sewing-kit', and the one dating back to Old English times, which now takes the form hussy.