I found myself in trouble with a role modification with a moodle/block:edit set to "Prohibit". The Admin thought that this permission was to prevent users from editing bloc settings, which seems logical , doesn't it?
In fact, this permission is used in blocks/moodleblock.class.php to define the function user_can_addto($page). It cheks if user has the capability 'moodle/block:edit' in a course level context for example. If not, the Add a bloc link is not displayed in navigation panel.
So... this is more a moodle/block:add capability? 🤔
Read you soon!