Oracle PL/SQL – How to Spool Data to Different Files Based on Condition

I am trying to get my way around something like this

declare
  filename varchar2(100);
begin
  for condition in (select fieldN from mytable) loop
    filename := 'data_for_value'||condition.fieldN||'txt';
    spool filename
    select some_data from some_table where fieldN = condition.fieldN;
    spool off
  end loop;
end;

In order to export different sections of data from a table onto different files but I just cannot get it right, I have read this and tried to do something similar in consequence:

set serveroutput on
set pagesize 0
set numwidth 2
set trimspool on

DECLARE
  filename varchar2(100);
BEGIN
  for field2 in (select field2 from blau2) loop
    filename := 'COST_TRAN_BASE '||field2.field2||'.txt';

    spool "run_query.sql" REPLACE

    select distinct 'spool "' || filename || '.txt"' || chr(10) ||
              'select field1, field3 from blau2 where field2 = ' || field2.field2|| ';' || chr(10) ||
              'spool off' cmd
    from blau2;
    spool off
    @"run_query.sql"
  end loop;
END;

But to no avail. I get the feeling that I am not allowed to use the spool command where I am trying to (I recently started working in the Oracle environment and the line between SQL*Plus and PL/SQL is still a bit blurry (you can kill me now for my ignorance :') ))

Am I just struggling against the impossible? D:

Edit:

with the initial help of Balazs Papp and my follow up investigations I came up with this system:
Using this file:

basic_script.txt
-----------------
SET PAGESIZE 0
SET TRIMSPOOL ON

spool RUN_QUERY.sql REPLACE

select distinct 'set heading off' || chr(10) ||
        'spool "BLAU2 ' || field2 || '.txt"' || chr(10) ||
                'select field1||chr(9)|| field3 from blau2 where field2 = ' || 
field2 || ';' || chr(10) ||
                'spool off'
    from blau2;

spool off
-----------

And this other file:

basic_script_condition.sql:
-----------
set feedback off
set trimspool on
set recsep off
set serveroutput on

col some_column new_value script_name
select decode((select count(*) from (select distinct FIELD2 from BLAU2)),1,'exit.sql','RUN_QUERY.sql') some_column from dual;
@&script_name
-----------

Calling them from command line in a simple .bat program:

basic_launcher.bat:
-------------
echo exit | sqlplus lmartin/lmartin @"BASIC_SCRIPT.txt"
echo exit | sqlplus lmartin/lmartin @"BASIC_SCRIPT_CONDITION.sql"
-------------

A script for exporting the different sections of data in the BLAU2 table is generated and executed only if there is data to export (if there is no data to export, basic_script would generate a .sql file with no rows selected as its content, possibly repeated N times, and the execution would fail, hence the basic_script_condition.sql controlling this behavior).