see https://github.com/vivien/i3blocks/issues/210

config

[test]
command=$SCRIPT_DIR/test.py
interval=persist
signal=1

test.py

#!/usr/bin/env python3

import signal
import time
import sys

# call reload() when RTMIN+1 signal is received
def reload(sig, hdl):
    print('reload')
    sys.stdout.flush()
    with open('signallog', 'w') as f:
        f.write("received signal 1")

signal.signal(signal.SIGRTMIN+1, reload)

# wait
while True:
    time.sleep(10)

test.sh

#!/usr/bin/env bash

SCRIPT_DIR=$(pwd) i3blocks -c config -vv > blocklog 2>&1 &
pkill -RTMIN+1 -f 'i3blocks -c config -vv'
sleep 5

$EDITOR blocklog