Editing a long backblast throws "msg_too_long" slack error

[jermo5]

User is able to successfully post a new backblast with long text, but when you attempt to edit the backblast (ex: add a missing PAX to the list), the following Slack API error is thrown:

The request to the Slack API failed. (url: https://www.slack.com/api/chat.update)
The server responded with: {'ok': False, 'error': 'msg_too_long'}