Using (Exporting?) pihole resources for opnsense DNS

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From what I understand Pihole is a great software for DNS queries.  So, what I don't understand is why people are using an external pihole along with their opnsense.  Since opnsense can be configured with multiple DNS programs, why not use them?

Why not configure the FW to download the block lists that pihole uses as aliases and set FW rules for denying traffic?

I've been looking on their site and help pages, but have not found the url they are using for the(se) block list(s). 

Am I missing something?

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