Blogs are different to other websites in that the content is (usually) not priorited or catagorised in any order other than descending date of posting. The architecture of the site is such that by default you cannot set pages up into different sections, or prioritise different pages over others. Therefore in order to find a specific blog post you will either have to have a contents page (which is time consuming to produce and maintain) or provide a Search option to trawl the history of posts.
A search field on blog sites would be easier to implement than on, say, an ecommerce site, because all that is being searched for is the textual content within historical blog posts (and not things like order numbers, product codes, staff contact details and you will not be expected to return page results in a weighted order of priority).
As with most things it would depend on the content of the blog and how organised you structure the content. My own personal blog is purely for film reviews, so I have included a search function to allow people to see if I have ever reviewed or mentioned a specific film in the past. I have used the same feature when visiting other similar sites so from personal experience I have found it to be useful.