It could be TWO issues here.
What are you trying to achieve, is urpath_regex appropriate? is srcdoman or dstdomain not enough? what about url_regex? If you are sure this is what u want, then;
1. note that URL-path regular expression pattern matching, leaves out the protocol and hostname and hence there could not be any urls matching your blacklists.
2. the order of your other ACLs matters. note that squid acls are read top down and when a match is found, processing stops. so if your black listed sites are finding a match (which allows them through) then your urlpath_regex will not be used.
If possible, attach your whole squid.conf and also a sample of the contect of blacklists.