It is just assumed that blogs have a special deep and intimate connection with the search engines, allowing them to rank above other sites with ease. That is not the case, blogs do not get special treatment because they are blogs.
A thread at Cre8asite Forums named How are blogs identified?. In that thread Ammon Johns and Barry Welford explain that it is because a blog is designed in a very search engine friendly manner, basically out of the box. You and I can build something just as search friendly as a blog, but these blogs are like that, out of the box. They have the keywords in the right places in the source code, they have frequent and fresh content through comments and frequent updates, and they get links really easily through trackbacks.
Also, as Barry W. said, they have much potential in search with the out of the box RSS functionality. I have been saying for a while, that RSS is going to be a major player in search. That is why I gave Yahoo! so much credit. But now it looks like many of the other engines are well into the RSS game.