An external class is a concrete class (instance of the Rule-Obj-Class rule type) that corresponds to a table in an external relational database, rather than to a table or view in the PegaRULES database.
You can create an external class and associated properties by using the Connector and Metadata accelerator or the New External Database Table Class Mapping button on the Data Model Classes and Properties landing page
To answer your question, No Pega won't create two table separate tables for a concrete class.
There isn't anything RDB or Blob table,
Pega saves the content of OOTB classes as BLOB with additionally exposed columns which are used either for logging or auditing or manually exposed column through the wizard to exploit better performance.
Pega suggests using RDB method only when OOTB retrieval Obj-Method didn't yield the business requirement such as calling stored procedure writing complex or custom query or SQL functions. Or for external tables, unless the Obj method is unable to perform what it requires. And external mapping could be done by "New External Database Table Class Mapping" functionality as suggested by my colleague Sudhakar.
Hope it answers your query. Kindly notify, if it does.