Neither DBA_SEGMENTS nor DBA_TABLES would give you the actual no. of blocks used.
Here is the work around to get the actual no. of blocks used.
This is written for Oracle8 and will not work for Oracle7 because the ROWID format is different, however it can be easily modified to use with Oracle7 also.
Code:
select count(distinct(substr(rowid,7,3)||substr(rowid,10,6)))
from :your_table_name;
Sanjay