'Blocks' includes 'empty blocks'.

so if you add up all of the 'blocks' and group it by tablespace,
then that is what extents are allocated in teh tablespace. anything left is overhead (a very small amount) or free space.

So it sounds like from your example you have used

46mb + sum(blocks from dba_indexes) * 8192

of your tablespace.

Yes you'll see that 771 change and exents are allocated and used up (assuming you re-analyze your tables!)

-John