Q:

Why would a phishing blacklist be hashed? 

I was planning to comment on the question at Finding phishing sites to certain domain and suggest searching Google's blacklist for keywords or substrings of a domain name in question. 
But then I found https://stackoverflow.com/questions/44663025/how-to-test-if-a-url-is-a-phishing-in-command-line-using-google-safe-browsing which says the blacklist is hashed, which makes it useless for the first question above.  There's a link to https://developers.google.com/safe-browsing/v4/urls-hashing which explains the hashing, but doesn't explain why the list is hashed. 
Why hashed?  What security problem does that solve?  It seems all hashing does is cripple the list's potential usefulness. 

A:

Why hashed?  What security problem does that solve? 

I could imagine several reasons for this decision:

Using the shorted hashes saves a lots of space. 
While it is possible to easily lookup specific URL's it is impossible to find out the whole list and then maybe reverse engineer parts on how Google creates this list in the first place.  This makes it harder to work around the blacklist in a generic way, i.e. create URL's which have the highest chance to not get accidentally detected. 
It takes efforts to create this list and Google might just want to protect their intellectual property.  

It seems all hashing does is cripple the list's potential usefulness. 

Usefulness for whom?  Note that this blacklist is intellectual property by Google and is obviously not intended to be open source.  If this would be Google's intention then 
they could simply publish the whole list and provide the tools needed to build the hashed version from it.  The same way you could ask why Cisco does let you query OpenDNS for free but does not offer the list of all domains considered malicious by OpenDNS for download.