Test case (database and connection charset is win1251):

create or alter procedure p_test
returns (
   rresult blob sub_type 1 segment size 80 character set unicode_fss)
AS
begin
 rresult= 'Тест'; -- text in cp1251
 suspend;
end

select rresult
from p_test
union
select rresult
from p_test

The field value is not transliterated. But if a sub-query is executed separately (without union), then the field value is transliterated properly.