Hide or make your text very small with css. One of my competitors is doing this and has outranked me in the serps for months. And his pagerank is pr 5.
I think the biggest experiments in black hat that need to be done is using linking techniques to try to damage a competitors site or rankings. There are still rumors that there may be vulnerabilities in google's algorithm that make it possible to do (some?) harm to a competitors site by simulating paid links or links from bad neighborhoods or spam-linking schemes. Would love to see that type of black hat scheme thoroughly tested to confirm or deny the rumors.
I think it is a good exercise for every white hat SEO to do. You can use the techniques without monetizing them to better understand how the algorithm is changing, then use your insights to create better whitehat sites.