Guest blogging has been losing its legitimacy for some time. While this once was a fantastic idea for blog owners and guest bloggers, what we've seen in the last few years are paid guest blog spots that are often irrelevant to the blog itself and sometimes even have spammy links that mislead readers.
Unfortunately for those folks, and maybe the legitimate guest bloggers too, is that Google is working hard to eliminate that kind of thing. Their latest update, called Hummingbird, was designed to filter through results as fast as possible to give the searcher links that are relevant to their search.
The Google Hummingbird update, unlike the former Penguin and Panda, completely renewed Google's search engine instead of changing the way search results were served.
As for guest blogging, what Google Hummingbird is trying to do is to more accurately tailor their search results to the queries. In order to have your site rank in Google's search results, you are going to have to make sure that each post is tailored to your audience.
For example, if your site is about peanuts, then you don't want a blog or guest blog about medications or car parts or anything unrelated and you will also need to make sure that your links are legitimately related to your content, preferably to vetted resources. Otherwise, Google Hummingbird may begin to rank your site lower in search results and your traffic will go with it.
You may also need to rethink your keyword strategy in your SEO endeavors. Google Hummingbird is trained to look for stuffed keywords as well as keywords that aren't related to the topic of the blog post, so you'll need to make sure that your use of them is moderate and that they are relevant to your post and your blog.
Essentially, what Google Hummingbird is attempting to do is to get website writers and bloggers to avoid the little tricks that some of them have been using in the past to get traffic using irrelevant links, keywords and content.
As long as you are industrious and vigilant with the information that goes out on your blog, you and Google Hummingbird will do just fine. Make sure that any guest bloggers also follow your formula to keep your site as high as possible in Google's rankings.