Oleg,

I believe that is correct that the block not erased is a bad block.  In Logic Loader version 2.4.14 you can view the number of bad block using the 'info mem' command.

Here is an example output from a SOM with no bad blocks.

losh> info mem

Configuration of /dev/nand0:
        Total Blocks:           4096
        Bytes Per Block:        16896
        Has Chunks:             yes
        Bad Block:              0
        Total Chunks:           131072
        Chunks Per Block:       32
        Bytes Per Chunk:        528
        Bytes Per Spare:        16

It is not unusual for NAND memory to have bad blocks. Some NAND memory even come with manufacturer-marked bad blocks. 

The specification for the NAND used on the i.MX31 allows up to 80 bad blocks.  Here is a link to an application note from ST Micro on bad block management. 

Here is another forum post on NAND recovery of bad blocks.  

Regards,

Brad