In the latest release, we changed the permissions on that file, removing the read permission. This prevents users other than Pi-hole from having access to the data contained in the file (for privacy reasons).
The command to use to tail this file is pihole -t. This not only live tails the file, but it color codes the output so you can more easily identify blocked domains.
If you want to search the pihole log, use sudo. Example:
pi@Pi-3B-DEV:~ $ sudo grep apple /var/log/pihole/pihole.log | grep query | tail -n5
Jul 8 08:03:01 dnsmasq[2668]: query[HTTPS] itunes.apple.com.edgekey.net from 192.168.0.135
Jul 8 08:03:01 dnsmasq[2668]: query[A] itunes.apple.com.edgekey.net from 192.168.0.135
Jul 8 08:03:01 dnsmasq[2668]: query[HTTPS] p16-buy.itunes.apple.com from 192.168.0.135
Jul 8 08:03:01 dnsmasq[2668]: query[A] p16-buy.itunes.apple.com from 192.168.0.135
Jul 8 08:03:01 dnsmasq[2668]: query[HTTPS] p16-buy-lb.itunes-apple.com.akadns.net from 192.168.0.135