A Blameless Postmortem is a tradition in modern DevOps Organizations.
The Postmortem as a debriefing of an incident (or accident or any other event that had negative impact on relevant processes or operations) with the goal to understand the event and prevent future occurrence (aka increase the resilience).
The Blameless part refers to a cultural aspect, that focuses on the contributing causes, not on the individuals involved. It is about the process, that needs to change, not the people, that are to be blamed.
The Blameless Postmortem should be scheduled as soon as possible after the event. In a sense, it is a time-delayed Swarm and Solve (as it pulls in multiple actors to the task of understanding / fixing / mitigating and issue, if in hindsight).
A definite sign for a broken company culture is blaming and finger-pointing in the context of a Postmortem.