Your statement:
select count(*) into :b0:b1 from dba_users;
...is functionally equivalent to:
select count(*) from dba_users;
...in SQL*Plus v11.2.0.3, as near as I can tell, as neither B0 nor B1 is populated from that SELECT, and it does not throw an error.
So, are you looking for Toad to ignore the INTO clause, like SQL*Plus does?