@butterfly : I am sure you meant to say by sending out newsletter to the people who are associated with your blog for some reasons. (readers/authors/subscribers, etc, )
A blogpost is something that you publish on your blog and that has more reaches than to a newsletter. A blogpost should be very informative, catchy and in detailed about covering the topic.
A newsletter is something that you send to people one by one. Writing a newsletter means writing something that don't let your user go off after a few lines but engage them to read thoroughly and so you can be more productive to supply the information you want to with the help of newsletter.
A blogpost is visible to everyone that will be a static, however you can make a customer newsletter for someone specific.