Instance with status BLOCKED and RMAN-04006, ORA-12528, Instance "testdb", status BLOCKED, has 1 handler(s) for this service...

Instance "testdb", status BLOCKED, has 1 handler(s) for this service...

Instance with status BLOCKED and RMAN-04006, ORA-12528 dont be panic or worry with this error:

Solution: Solution is very simple.

When an instance within open mode: NOMOUNT, 
you see on Listener status the associated instance is listed as follow:

Instance "testdb", status BLOCKED, has 1 handler(s) for this service...

when db will become in open mode you'll not find furhter this error.

..other case, when you use RMAN to duplicate the database and try this connect:
rman target sys/password@source_db auxiliary sys/password@duplicated_db

RMAN-04006: error from auxiliary database:
ORA-12528: TNS:listener: all appropriate instances are blocking new connections


Solution here: Don’t try connect auxiliary duplicated_db over listener. 

Try with this:
set oracle_sid=duplicated_db
rman target sys/password@source_db auxiliary /