The purpose of a postmortem is not to produce a document. It is to make the same incident less likely, and to spread what one person learned to everyone else.
Why blameless matters practically
The practical argument is stronger than the cultural one. If a postmortem can end in someone being blamed, people will shade what they report, and the omitted details are precisely the ones you need. Blameless is about getting accurate data. The test is simple: could a competent, well-intentioned person have made the same decision with the information available at the time? Almost always yes, which reframes "Ali deployed without testing" into "our deploy path allowed an untested change to reach production."
A structure that works
Summary — three or four sentences, written last. Impact — concrete and quantified. Timeline — the core. Contributing factors — plural, deliberately. What went well — not filler. Action items — with owners and dates.
The timeline is where the value is
Timestamps, in order, of what happened and what people did, including the things that turned out wrong. A good timeline reveals gaps a prose summary never would: five minutes between a problem starting and detection is a monitoring gap with a number attached; fourteen minutes between acknowledgement and suspecting the deploy is evidence that deploy information was not in front of the responder. Include the dead ends.
Contributing factors, not root cause
"Root cause" implies a single point. Real incidents have several factors and therefore several possible interventions. If you write "root cause: bad deploy," your only intervention is "be more careful."
Action items that survive
An owner who is a person, not a team. A date. Small enough to actually finish. Prefer three items that get done to eleven that get filed, and put them in the same tracker as normal work.
The habit that makes it cheap
Write the timeline during the incident. Whoever is responding drops rough timestamped notes as they go. Afterwards, cleaning them into a timeline takes ten minutes instead of an hour of reconstruction, and it is accurate, which the reconstructed version never quite is.