Preserving  tags inside a  tag in a Redactor rich text field
Would you consider an alternative Matrix based approach where one rich text field was used for article-copy and another separate rich text field, used for the blockquote?

So: Matrix field: "Article blocks"


Block 1: "Article"
Block 2: "Quote"


e.g. (screen shot from a current project...)



That way you could leverage Matrix functionality and allow more layout control as well - such as alignment etc - to the blockqoutes, and maintain your paragraphs as well.