A slashtag is essentially a filtering tool to restrict search results. This offers up some interesting capabilities. For example, in Blekko you could enter the search term "healthcare" just as you would in any search engine. But with Blekko, you can add the slashtag "/conservative" after "healthcare" to see only the conservative perspective on healthcare.
You're probably already a step ahead of me and asking that all-important question "How do it know?" That's where it gets interesting. A slashtag represents a list of websites and web pages. Humans will build and maintain the lists associated with each slashtag. Blekko intends to build some popular ones, but ultimately it wants its users to build slashtags, either for public or private use. Long story short, Blekko is a horizontal search engine that offers as its primary feature a vertical search capability.
Keep in mind too that Blekko is a general search engine, so it is not restricted to news. It's entirely possible that you could enter "American Refractory Corp. /companies" into Blekko to have it only search public company databases.
The weakness I see in Blekko is its approach to slashtags. Apparently, anyone can make one, anyone can name one, they won't all be equally good, and they won't all be maintained. Until Blekko nails these issues, it's a curiosity and a tool for power searchers.
What's really interesting , though, is the larger statement that new search engines like Blekko are making: the game in search has shifted to filtering and limiting search results. Less is more in the next generation of search. It began with vertical search; now Blekko is offering what might be called multi-filtered vertical search. The ultimate challenge of course is to get to fully structured data that can be searched and filter with precision, a business data content producers know very well.