IfxCommand cmd = conn.CreateCommand();
            cmd.CommandText = "INSERT INTO test_blob (blob_val) values (?)";

            IfxParameter p1 = new IfxParameter(null, IfxType.Blob);
            p1.Value = DBNull.Value;
            //p1.Value = IfxBlob.Null; //Тоже не работает
            cmd.Parameters.Add(p1);

            cmd.ExecuteNonQuery();

            //ERROR [HY000] [Informix .NET provider][Informix]Illegal attempt to use Text/Byte host variable.