A simple grouping will do the trick:

SELECT ( LENGTH(blob_column) / 32 ) * 32 AS blob_size,
      COUNT(*) AS row_count
FROM   ...
GROUP BY ( LENGTH(blob_column) / 32 ) * 32
ORDER BY 1

If you want to have exactly your above formating, a little bit of polishing
is required:

SELECT RTRIM(CHAR(blob_size - 31)) || '-' || RTRIM(CHAR(blob_size)),
      row_count
FROM   ( SELECT ( LENGTH(blob_column) / 32 ) * 32 AS blob_size,
               COUNT(*) AS row_count
        FROM   ...
        GROUP BY ( LENGTH(blob_column) / 32 ) * 32 ) AS t
ORDER BY 1