If I'm reading it correctly, it seems to check if IP addresses are blacklisted (rather than domains), and returns true if the domain doesn't resolve (which isn't the same thing as being blacklisted).
I could be reading it wrong though, I'm a horrible software developer :P
At the moment though it seems like UVS generates some confusing error messages about blacklisted IPs when it just can't resolve the DNS.
https://github.com/matrix-org/matrix-user-verification-service/blob/dddc364797f168a3dbd0c440d137346c2766c728/src/utils.js#L131
If I'm reading it correctly, it seems to check if IP addresses are blacklisted (rather than domains), and returns true if the domain doesn't resolve (which isn't the same thing as being blacklisted).
I could be reading it wrong though, I'm a horrible software developer :P
At the moment though it seems like UVS generates some confusing error messages about blacklisted IPs when it just can't resolve the DNS.