ERROR: database "dbname" does not exist
Your database was created using double quotes so its name is now case-sensitive. Therefore, you now need to always use double quotes when referring to it:

drop database "Blog_test";


More details about quoted identifiers (a database name is an identifier just like a column or table name) can be found in the manual: http://www.postgresql.org/docs/current/static/sql-syntax-lexical.html#SQL-SYNTAX-IDENTIFIERS