Can you let us know if you are using this in the Case or is it used in any section which is not part of a case? Because i see you are using a temporary page as context for the property, the error might get set in the temporary page only and if you are using this validate rule as a part of the case and expecting the Error to occur in pyWorkPage it wont work since you are giving a different context for your error. Please check the clipboard if the error is getting set on “TempBlackListPage” if yes then your validate rule is working fine its just that you need to set this error on pyWorkPage through and activity/DT instead of invoking this in Validate of Flowaction.
Note: Please check if this property is available in pyWorkPage, so that you can directly use the validate rule