Both are old-fashioned words for a bad, dishonest person. I've only seen the word blackguard in old books, never heard anyone use it in everyday speech.

The word 'scoundrel' is a lot more common than 'blackguard.' The latter is more used in literature especially in UK, India but almost never in the US/Canada. Also note that the pronunciation of 'blackguard' is BLAG-GARD.

scoundrel means a man who treats other people badly, especially by being dishonest or immoral blackguard means a man who is dishonest and has no sense of what is right and what is wrong. I think they’re synonyms, and there are no so many differences.