Hey, there are a bunch of ways to identify guest blog opportunities, and some sites / services etc, but I tend to think a lot of that is played out to some extent and if they are accepting content from any Tom, Dick and Harry then how much value will your post have from a pure SEO / exposure / link perspective AND for how long?
I tend to think, rather than looking for Blogs that accept guest posts, look for the best Blogs in your target niche and try to spark up a relationship with the owners. Then, try to find a gap or someway you can contribute always focusing on the 'what is in it for them' angle AND be focused on doing it for the exposure you can generate.
Sure, you want a link or two but make sure it is super valuable, super relevant to their core audience and then that post may pick up more links, shares etc from the current audience and subsequently, the links to your site and more importantly the exposure will have more value.
Forget about volume, think about quality and focus on getting links that your competitors can't just pull up in OSE and copy.
Hope that helps!
Marcus