This reply may be too late for you, but for anyone in future can refer to this.
If you are a Joomla developer you must be knowing about the Plugins and triggers they work on, for any change in User block, update, delete etc. Joomla triggers events from User Plugin( refer here => User plugin events)(Joomla Plugin Events)
For your case, you can use either onUserBeforeSave or onUserAfterSave
better will be to use before save as it will be easy for you to find the change in the value.
public function onUserBeforeSave($oldUser, $isnew, $newUser)
var_dump($oldUser, $isnew, $newUser);
The $oldUser gives the old values and $newUser gives the new values
you need to apply proper condition for your operations only when the condition matches the user block.
Also, onUserBeforeSave method if you return false the user will not be updated but, if you use the onUserAfterSave you will get the updated user object and you have to use your own logic to find the change in value.
I will request all the community users to provide answers to their questions if they have found a solution to the query they report.
We can learn and grow together.