In movies, TV shows, and other forms of entertainment, there are often good guys and bad guys. A blackguard is a bad guy. Blackguards are up to no good.
Originally, a blackguard was a type of servant dressed in black, but the meaning evolved to mean a person who is villainous at heart. This is definitely an old-fashioned word you're most likely to see in an older story or play. A blackguard likely won't be guarding anything, but they probably will be doing something evil.