The optimization process and recommendations:

1. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.

Optimal indexes for this query:

CREATE INDEX pg_locks_idx_transactionid_pid ON "pg_catalog"."pg_locks" ("transactionid","pid");
CREATE INDEX pg_activity_idx_pid ON "pg_catalog"."pg_stat_activity" ("pid");

The optimized query:

SELECT
        bl.pid AS blocked_pid,
        a.usename AS blocked_user,
        ka.query AS current_or_recent_statement_in_blocking_process,
        ka.state AS state_of_blocking_process,
        now() - ka.query_start AS blocking_duration,
        kl.pid AS blocking_pid,
        ka.usename AS blocking_user,
        a.query AS blocked_statement,
        now() - a.query_start AS blocked_duration 
    FROM
        pg_catalog.pg_locks bl 
    JOIN
        pg_catalog.pg_stat_activity a 
            ON a.pid = bl.pid 
    JOIN
        pg_catalog.pg_locks kl 
            ON kl.transactionid = bl.transactionid 
            AND kl.pid != bl.pid 
    JOIN
        pg_catalog.pg_stat_activity ka 
            ON ka.pid = kl.pid 
    WHERE
        NOT bl.GRANTED