Oracle: Listener show service as blocked/ORA-12514

lsnrctl status:
Service "BLOK" has 1 instance(s).
  Instance "BLOK", status BLOCKED, has 1 handler(s) for this service

Solution:
Assuming your listener name is LISTENER(default), add the following entry with correct home,global_dbname,sid_name.

SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (GLOBAL_DBNAME =ALOK)
      (ORACLE_HOME =/u01/app/oracle/product/11.2.0)
      (SID_NAME = ALOK)
    )
    (SID_DESC =
      (GLOBAL_DBNAME =BLOK)
      (ORACLE_HOME =/u01/app/oracle/product/11.2.0)
      (SID_NAME = BLOK)
    )
   )

Post above change: restart the listener.

Service "BLOK" has 1 instance(s).

  Instance "BLOK", status UNKNOWN, has 1 handler(s) for this service...

The command completed successfully

Also solution to 

ORA-12514: TNS:listener does not currently know of service requested in connect descriptor

above while creating auxiliary database via RMAN.