What does it mean for a SQL Statement to be blacklisted?

The 19C Database Administrator's Guide, in the section describing the Automatic Indexing feature (https://docs.oracle.com/en/database/oracle/oracle-database/19/admin/managing-indexes.html#GUID-082972AD-1866-411A-8250-9… ), says this (emphasis mine):

The invisible auto indexes are validated against SQL statements.

If the performance of SQL statements is improved by using these indexes, then the indexes are configured as visible indexes, so that they can be used in SQL statements.

If the performance of SQL statements is not improved by using these indexes, then the indexes are configured as unusable indexes and the SQL statements are blacklisted. The unusable indexes are later deleted by the automatic indexing process.