Is aggressive and careless black hat SEO on behalf of an enemy a viable (albeit possibly illegal) strategy for getting them blacklisted from Google?

There are a lot of speculations about whether you can actually get your competitors banned from Google by building 1000s of spammy links to their website. However, I am not sure if this works at all. Based on my experience, most of the time Google actually ignores such links. However, I think it is because they first analyze the current link profile of the website. If you have lots of spammy links and your competitor just build 1000 and reported you, then you have a good chance of getting some kind of a Google penalty. On the other hand, if you are getting links from quality, relevant websites. Google is more likely to ignore all spammy links they find after, the report. Finally, it is nearly impossible for the search engines to monitor every website, so there are still some quite old black hat strategies that atually work. In other words, there is a chance to boost your competitors rankings, instead of get them penalized.  More info at: http://www.seopalbg.com/blog/the-nature-of-black-hat-seo/ All this is the most unethical way to fight the competition and I am pretty sure Google and the other search engines, have pretty handy filters and algorithms to fight this.

It's highly possible if the site isn't recognized as a brand in google, is relatively new, and has very low domain authority. See http://www.ericwagner.org/do-ceos-get-speeding-tickets/ Of course it is highly unethical, and way too likely to backfire.

No -- Aside from the fact that it's completely unethical and probably illegal, I would assume it could cost you a very large amount of money for this to work. It would be a pretty sad approach to marketing and there's no clear evidence on how well it would work. On top of that, they would mention that they were sabotaged and work with Google to get their site back in the index as soon as possible.

I think this is highly dependent on the  competitor and really what you are attempting to gain. I'm always of the  opinion that if you do something better than your competitor it will be  money better spent than attempting to "poison" their link profile or  something of that nature.  Assuming that you are hiring someone to do "black hat seo" and point  many links from bad neighborhoods to your competitor, it is likely that  those links will simply be ignored. While I've never seen this in action  or anything like that, it's what I've heard on many conference panels.