I see two options for you.
Option 1 - changing dnsmasq option on the fritzBox
I do not own a FritzBox but if like many routers it is usinf dnsmasq, you can usually use one of the dnsmasq option related to DNS rebind, see the man page, or here is the essential part:
Exempt 127.0.0.0/8 from rebinding checks. This address range is returned by realtime black hole servers, so blocking it may disable these services.
Do not detect and block dns-rebind on queries to these domains. The argument may be either a single domain, or multiple domains surrounded by '/', like the --server syntax, eg. --rebind-domain-ok=/domain1/domain2/domain3/
You can see that the option
--rebind-localhost-ok is kind of made for DNS black hole like you want to create. However, you cannot really use this option to answer something else than a loopback address. So it might not fit to your need.
Note that I have no clue if using a FritzBox, you can override dnsmasq options somewhere.
Option 2 - changing your DHCP DNS setting on the router
If you cannot change the parameters of dnsmasq on your FritzBox, then you still have another option. In your DHCP settings on your FritzBox, set there the DNS entry to point to your Raspberry Pi. What you have currently is a client request a DHCP address to your router, the router replies with an IP, gateway (the router IP to access the internet) and a DNS (which I bet is the router IP again). Now when your client query a domain name, this is forwarded to your router DNS forwarder, which forwards it to your Pi, and then back.
If you do the suggested change, your clients will get during the DHCP transaction the DNS IP from your Raspberry Pi directly. So you won't get the rebind warning anymore :-)
PS: there is also a project called Pi-Hole which could help you.
PS2: edit after checking the Pi-Hole project, someone suggested in an issue my option 2 as a resolution for using Pi-Hole and a FritzBox. https://github.com/pi-hole/pi-hole/issues/1271#issuecomment-282295061 so I guess option 2 is the recommended solution.