I've missed that feature to be able to set a separate ringtone for "unknown" calls. I used to have the song that was used for the theme on CSI. It was perfect because it was the part where it goes "who are you? who who, who who" lol.
Only thing i can think of that would be a work around would be to assign a ringtone to every contact you have in your phone book and then have the default ringtone be something that would identify calls from numbers not in your phone book or that are blocked. Annoying and not very practical though.