-blackhats could fight back by:
-creating a program that just simply searched and clicks on google's ads until the website gets banned by google automatically
-creating a program that is like a toolbar that is distributed by illegal downloaded files ie music....the biggest thing on the internet which billions do.
This program would simply then if user clicks on google just reverts the searches so top searches are page 100 and rubbish search results are page 1
Google+ a program that automatically scrapes users emails and just simply trys to login until user get that sms verification thing if this is done on a mass scale users will give up on google + resulting in mass loss for google.
Im sure their are many other ways but the internet is dynamic beat google show google it is!
But i guess the reason blackhats dont fight back is money and it doesnt pay well it would in the long run as google would be forced into changing and bing would get richer and afford a good adsense alternative.... more comptition meaning even higher rates..........