ob2=> select counters->'_blog' from _site; 
 ?column? 
----------
 
(1 row)

ob2=> UPDATE _site SET counters=counters||('_blog' => (select (counters->'_blog')::int + 1 from _site));
UPDATE 1
ob2=> select counters->'_blog' from _site; 
 ?column? 
----------
 
(1 row)