One of them, the blog, is a public airing of your thoughts and dirty laundry, where a diary is supposed to be a private account of your life. A blog has turned into more of a way to advertise yourself and your products or ideas and less about sharing personal information, but when they started out it really was an exhibition of your thoughts and a "public diary" if you will.
I think a diary is a record of your life, what happened today, your personal stuff etc etc A blog is more about your opinions on things, ideas about stuff like news items etc
diary >A book for use in keeping a personal record, as of experiences. blog >is a website,usually maintained by an individual, with regular entries of commentary, descriptions of events, or other material such as graphics or video. Entries are commonly displayed in reverse chronological order. "Blog" can also be used as a verb, meaning to maintain or add content to a blog.
Theres a big difference but it isn't very noticeable. A diary is more private and usually contains a persons secret thoughts and desires while a blog is public.
diary-to note your secret,personal use and private property. blog-you can post anything under the sun its for everyone.