DNS requests are created by clients, Pi-hole is just receiving them.
Accordingly, you'd have to control your client's behaviour (as you've achieved by blocking IPv6 requests).
You have several options to do so:
a) Configure your router to not advertise an IPv6 DNS server address at all.
It's important that it doesn't offer its own IPv6. Depending on your router, that may not be configurable.
b) configure a ULA address for Pi-hole.
This may or may not reduce the amount of queries posed via a public IPv6 address, depending on a client's IPv6 prefix policies
c) change a client's IPv6 prefix policies to prefer IPv6 ULA or IPv4 over public IPv6 in your local network. This isn't supported on every device (e.g. IoT).
d) configure your client to stop creating temporary IPv6 Privacy extension addresses and create a local DNS record for the public IPv6, overlaying your ISP's generic names.
As those addresses are meant to improve privacy by regularly changing your IP address (e.g once every two hours), I do not recommend doing so.
e) configure your router to make use of your Pi-hole machine's link-local IPv6 address (range fe80::/10).
This is only on an option as long as there are no L3 switching devices in your network, as link-local addresses are accessible by same link or same network segment devices only (which can be assumed if all devices are connected directly to your router).