A blog is not strictly PR.
It may have aspects of PR.
It may work with PR companies.
But a blog is not looking for content.
It is especially not looking for photos. (Unless they are of Wil Wheaton collating paper.)
A blog is almost always an expression of someone's thoughts, whether they be personal or professional.
The only time a blog will act like big media outlets scouring for stories, is when they have become big media outlets scouring for stories.
A blog may have advertising. On the side. That is paid for.
But a blog almost always will never have content within a post that is placed like paid advertising. And if they do, they will declare it. And it is paid for.
A blog is a brand new frontier somewhere between traditional free PR and traditional paid advertising.
Since we all like charts, here is a chart to show your clients to help you explain what I am talking about.
Do not be offended when I tell you I can't work on your campaign for free.
Whether I am paid in dollars, or product, or experience, I have to make it worth it for me.
Do not be offended when I tell you I can't work on your campaign because it does not match my blog.
I don't want to alienate those readers you covet so much, but more importantly - I want to feel good about what I am posting.