The target database must run in ARCHIVELOG mode and be open or mounted with a current control file.

If the target database is a standby database, then it must be in a consistent state, recovery cannot be in session, and the backup must be older than the corrupted file.

The backups of the data files containing the corrupt blocks must be full or level 0 backups and not proxy copies.

If only  backups exist, then you can restore them to a nondefault location on disk, in which case RMAN considers them data file copies and searches them for blocks during block media recovery.

RMAN can use only archived redo logs for the recovery.

RMAN cannot use level 1 incremental backups. Block media recovery cannot survive a missing or inaccessible archived redo log, although it can sometimes survive missing redo records.

Flashback Database must be enabled on the target database for RMAN to search the flashback logs for good copies of corrupt blocks.

If flashback logging is enabled and contains older, uncorrupted versions of the corrupt blocks, then RMAN can use these blocks, possibly speeding up the recovery.

The target database must be associated with a real-time query physical standby database for RMAN to search the database for good copies of corrupt blocks.