Ufeliciano,
I am weary of aliasing in PostgreSQL, the following query gave correct results:
select sde.block_groups.tract90, sde.census_tracts.tract90
from sde.block_groups , sde.census_tracts
where sde.st_intersects (sde.census_tracts.shape, sde.st_centroid(sde.block_groups.shape));
Sincerely,
Kasia