There is nice BLR (binary) to BLR (text) decoder in Firebird itself. You may call it from ISQL:
SQL> set term !;
SQL> create procedure p1
CON> as
CON> declare n integer = 1;
CON> begin
CON> n = n * 2;
CON> end!
SQL> set term ;!
SQL>
SQL> set blob all;
SQL> commit;
SQL>
SQL> select rdb$procedure_blr from rdb$procedures where rdb$procedure_name = 'P1';
RDB$PROCEDURE_BLR
=================
1a:3
==============================================================================
RDB$PROCEDURE_BLR:
blr_version5,
blr_begin,
blr_message, 1, 1,0,
blr_short, 0,
blr_begin,
blr_declare, 0,0, blr_long, 0,
blr_assignment,
blr_literal, blr_long, 0, 1,0,0,0,
blr_variable, 0,0,
blr_stall,
blr_label, 0,
blr_begin,
blr_begin,
blr_assignment,
blr_multiply,
blr_variable, 0,0,
blr_literal, blr_long, 0, 2,0,0,0,
blr_variable, 0,0,
blr_end,
blr_end,
blr_end,
blr_send, 1,
blr_begin,
blr_assignment,
blr_literal, blr_short, 0, 0,0,
blr_parameter, 1, 0,0,
blr_end,
blr_end,
blr_eoc
==============================================================================