It's all cobblers as long as you remember it's about quality and relevance. If you have a blog about ice cream, then having someone guest blog about Forex trading is clearly spam. Likewise if your blog is nothing more than a collection of guest blogs of varied subjects, then that looks more like an article directory of yesteryear.
But having occasional on topic and good quality guest posts on your blog are absolutely fine. Problem is finding them.
Assuming your blog is about bulldogs, then you need to become the go to resource about bulldogs. Invite other bulldog experts to do a post, maybe breeders talking about things to look out for in puppies. Maybe you could invite a vet or expert to write about typical health conditions of the breed. Can you contact any competition winners, past and present. Maybe they could write about the excitement of winning a trophy, or telling owners how to go about showing for themselves.
If its your blog, keep control of the content and direction. Once you have obtained credibility and and popularity, people will naturally approach you and ask if you can publish their posts.
No magic bullet, just hard work reaching out and forging contacts, but use social media to find your target audience and approach them. Worse they can say is no 
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Hi Steve
as im not a business but a collection based website I understand that my readers want variety but not to far away from the premise of collecting, if someone wanted a blog guested that was not topic friendly it would be added to the end of my blog with a small bio that way folks can either read or jump over to the galleries etc and not used as a published blog parsay. I've been waiting months for my new website the first designer procrastinated and it wasn't till last October I had the new one up and live.
Maybe im just being inpatient I mean everything has come good but in its own time organically and as its start of new year things go at a snails pace, As the collection is museum listed as object form I'm trying to reach out to collectors for blogging the idea of asking show winners etc or breeders is a good idea in theory but if you were in the bulldog world you would know it's very political so I stay away from that side and the breeds darker days.
i learned to swerve these topics but as I know others maybe opinionated or want a say I have created a forum for folks that wish to chat actual bulldogs
The museum have afforded me the luxury of a PA so I may ask her for some practical help in creating more contacts there's many a demographic that could be seen as topical I just lack certain social abilities to get this moving forward myself .
