Honestly? Whenever I've shot off emails to people as introductory emails it has always been fairly informal but always targeted.

The best advice I can give is make sure you know who you're writing too, what their blog is about. Perhaps include a link to a relevant article in their blog (always use it in plain text form, bloggers love having their ego's stroked)

If you need an example, I guess a very rough one is something like.

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Hi there .....

My name is ..... and I'm from ......

I've been reading your blog for ..... months and an article of yours that caught my attention was the one about .....

(link to article in plain text form)

My company does ..... and this is along the same lines as ......

I thought this might be of interest to your readership.

Anyway, thanks for taking the time to read this email. If you would like to get back to me my contact details are in the footer.

Thanks!

Name