As velociraptor said it is best to use a resolver that block adult content and not custom lists because every days hundreds of new sites/names appear.
Another reason is that actual adult blocking lists are so big that at this moment as it is pihole web interface will not handle that amount of data and will crash web interface when you try to check the logs.
I am using OpenDNS with customs settings for children ( porn + other categories )... it is free for home use.
edit.
In addition to that you can also force safemode for google/bing...
have a look here