Thesius "blaggard" voice line is subtitled "Blackguard"

[YokoZar]

This occurs in at least 3 or 4 different voice lines he has. They are different words.

[C4RNIVOR3]

That's not an error, that's actually how it's pronounced.

black·guard
/ˈblaɡərd,ˈblaˌɡärd/

noun
a person, particularly a man, who behaves in a dishonorable or contemptible way.
verb
abuse or disparage (someone) scurrilously.

[YokoZar]

"blaggard" is a completely acceptable spelling of the term

[C4RNIVOR3]

You insisted they are different words, the point is that there's no error in the dialogue.

https://www.merriam-webster.com/dictionary/blackguard
https://dictionary.cambridge.org/us/pronunciation/english/blackguard
https://en.wiktionary.org/wiki/blackguard#English

It's largely a dialect thing whether the -ck sound is prominent or not but blaggard is the generally accepted way of pronouncing blackguard.