Google’s John Mueller said on Twitter that they treat the content within quotes and blockquotes the same as other standard content on the page. So if you are quoting someone else, putting that in quotation marks or blockquotes, Google won’t be devaluing that content.
Here is the context of this response from John:
We generally treat “standard” text the same as quoted text on a page.
— 🍌 John 🍌 (@JohnMu) October 10, 2018
Of course, John Mueller used the word “generally,” which he generally does around topics like this. John wrote “We generally treat “standard” text the same as quoted text on a page.”
Forum discussion at Twitter.