IMO, if you're trying to make inroads into search market share against a juggernaut like Google then underdogs banding together makes a lot of sense. It's the "enemy of my enemy is my friend" model. In a way, Yahoo went this route with Microsoft's Bing search engine, except they're letting Bing lead the charge completely. If either Blekko or DDG began to grow too big for the partnership they could always pull back. Actually, that's what Google did, since they used to supply some search results for Yahoo.

DuckDuckGo's long term perspective is getting to the best search UX. We use a lot of APIs to do that, and now blekko is one of them.

They are theoretical competitors but I'm guessing the overlap between their user bases is really small.