Your router most likely will allow you to set ITS DNS to your local PiHole. So the DHCP will hand out your home router's IP as DNS server, but that in turn will

Your router most likely will allow you to set ITS DNS to your local PiHole. So the DHCP will hand out your home router's IP as DNS server, but that in turn will access the PiHole as DNS server. Your chromecast will NOT use your DHCP supplied DNS settings, it goes straight to google's.