Blameless Post-Mortem
Blameless Post-Mortems are adopted as part Google’s Site Reliability Engineering framework. The concept has existed for sometime as part of traditional Service Operations. Its use within SRE shows its adoption in "DevOps” too. In this context, post-mortems are used to analyse the causes of operational failures and to decide how to improve the situation. Blameless post-mortems adopt the essence of the retrospective prime-directive by exploring problems without reference to personal responsibility thereby avoiding blame.
In A Nutshell
Blameless post-mortems are the service operations equivalent of retrospectives found in Scrum. They seek to identify the root causes of failure without attributing blame. Having identified root causes, actions can be planned to avoid the problem recurring.
Originating in an operational context, blameless post-mortems suffer from an obsession with failure and learning from failure. There is very little reference to finding the underlying causes of conspicuous successes so that these can be repeated. In contrast most retrospective techniques seek to identify successes and failures.
Of course, there is no reason why blameless post-mortems cannot be used to study success as well as failure. The bias to failure probably originates in the culture of operations. If things go well, we don’t need (or have time) to improve; if things go badly we need to act.
Another significant difference to a retrospective is the scope of a blameless post-mortem. Retrospectives explore all opportunities and problems faced by the team, not just those associated with ways of working. Whilst it is possible that a blameless post-mortem could identify the root cause of a problem as lying within the behaviour of a team (for example), the focus on ways of working makes this relatively unlikely.
References
Description of blameless post-mortem as part of Google SRE
Smartsheet pages on blameless post-mortems, including a table showing differences from traditional post-mortems
Atlassian’s guidance on blameless post-mortems