You can define the list of BlackList users in the BlackListDefinition security rule. The security rule uses the Define Symbol action to create a list of users that view masked data. Separate user names with a comma.
If you want to mask data based on the users in the BlackList folder, enable the BlackList folder and disable the WhiteList folder.
For example, you have 100 users. You want to mask data when 20 of the users access the database. Enable the BlackList folder and disable the WhiteList folder. List the 20 users in the BlackListDefinition rule. Dynamic Data Masking will mask data when a request is sent from one of the users in the BlackListDefinition rule.