When you use the Slack → Update a message with Message Type: Blocks the format for the parameter isn’t what the user would expect. Instead of take the actual blocks e.g.
[
{
"type": "section",
"text": {
"type": "mrkdwn",
"text": "This is a *markdown* section."
}
}
]
it requires an object which includes the blocks element e.g.
{
"blocks": [
{
"type": "section",
"text": {
"type": "mrkdwn",
"text": "This is a *markdown* section."
}
}
]
}
This is alluded to if you have Fixed selected by the description, but this is not present with Expression as can be seen below.
As the API has a fallback text from the Notification Text option, if blocks can’t be parsed no error is returned, compounding the issue, as the user has no idea the format of the blocks parameter they passed is incorrect.
A hit for when Expression is selected would help avoid much head scratching.