[hsparikh]

As someone learning web development, I'd love to get some insights into how one could build this.

[lubujackson]

Not knowing anything about what they do, the hack-ish way you could do it is to use Google CSE (custom search engines) to add a list of negative domains. Where to get the list of top 1 million domains? Probably from Quantcast here: http://www.quantcast.com/top-sites-1