If I put up some blackhat seo subdomains, and things went horribly wrong - would this damage the rankings for my main domain?
Last time i killed subdomains with black hat experiments no it didnt effect the other subs or primary domain. If you go generating subs on the fly based on search queries and end up with a 5 billion page site like what happened late last year, well yes they will get deindexed by manual review. But honestly i would just buy an info for 99c and kill that.