What is the origin of the phrase "bad blood"?
Solution 1:
From what I researched: Dictionary.com states that the phrase "bad blood" was first recorded 1815 to 1825, and this seems to be the most accepted time from when I cross-referenced this.
Dictionary.com
However, there are people who can give examples from even earlier than this. Therefore, I think Merriam-Webster is correct, or quite close. Check out this link to a discussion I found.
Word Wizard
Hope this helps!