Have Pi-hole resolve hostnames of DNS clients, and report by hostname rather than IP, then DNS queries should follow the path:

client -> Pi-hole -> pfSense -> Internet

Navigate to System → General Setup.

In System:

• hostname: pfSense.
• domain: localdomain.

In DNS Server Settings:

• DNS Servers: Enter values for DNS Servers.
• DNS Server Override: Not Checked:
• Disable DNS Forwarder: Checked. This forces the firewall to use the DNS servers entered above.

Navigate to Services → DNS Resolver → General Settings.

In General DNS Resolver Options:

• Enable: Checked. This enables the DNS Resolver.
• Network Interfaces: Select the Network Interface to apply. LAN and localhost.
• DNSSEC: Checked.
• DNS Query Forwarding: Not Checked.
• DHCP Registration: Checked.
• Static DHCP: Checked.

In Host Overrides:

• Add systems with static IPs as Host Overrides.

Navigate to Services / DHCP Server.

Select the Interface to configure, such as LAN.

In General Options:

• Enable: Checked.

In Servers:

• DNS servers: Enter the IP address of the Pi-hole.

In Other Options:

• Domain name: Enter the same domain name as that specified in System → General Setup.

Navigate to Settings → DNS:

In Upstream DNS Servers:

• Custom 1 (IPv4): Enter the IPv4 address for the LAN interface on your pfSense.
• Custom 3 (IPv6): (Optional) If using IPv6 on the pfSense, then populate the IPv6 address for the LAN interface on your pfSense.

In Advanced DNS settings:

• Never forward non-FQDNs: Not Checked.
• Never forward reverse lookups for private IP ranges: Not Checked.