Where you can find Oracle Block Details

• DBA_SEGMENTS

                 –HEADER_FILE
                 –HEADER_BLOCK

• DBA_EXTENTS

                –EXTENT_ID
                –FILE_ID
                –BLOCK_ID

Both can be used to determine starting blocks of Index segments.
ASSM set to manual: Add 1 to BLOCK_ID to find Root Block
ASSM set to auto: Add 3 to BLOCK_ID to find Root Block (can vary)