A Retrospective is meant exactly to address this kind of issues around team dynamics (for the product feedbacks you have the Sprint Demo/Review).
If the team member persists in his/her behaviour, even after having being reminded of the Prime Directive ("Regardless of what we discover, we understand and truly believe that everyone did the best job they could") then the team’s scrum master needs to take action. Blaming games cannot be ignored nor tolerated.
You mentioned that it was not always the same, so probably something happened to ignite this behaviour and the team member does not feel comfortable to speak about it in public.
The scrum master should start by addressing the team member privately to discuss their concerns.
Possibly it was about private issues or only with another specific team member and could be solved individually or in a smaller group. Sometimes it helps a lot just to listen and acknowledge the concerns.
Only following private discussion and if the team member’s reservations ARE a topic of discussion for the entire team, then the team can be involved by discussing them during the next retrospective. This allows the team to offer their support.
If there is still no change in the team member’s attitude, then this needs to be escalated to his/her direct manager.
If still no change can be achieved, then the team member could be assigned to another team, with different members or possibly a non-agile team/project if the concerns were about this team member feeling to be forced out of his/her comfort zone.