Writing a blog tends to be a bit different than writing a “formal” book exactly because blogging lends itself to informality. When blogging you can write as if composing an email to a friend.
Blogs began as live journals, and many people considered them streams of consciousness—simply unedited thoughts thrown together and published. You don’t want to go to that extreme. You will want to carefully choose your words and edit your copy. You want to put your best words forward.
Even if you are blogging on a technical subject or on a scholarly topic, you can approach writing your blogged nonfiction book in a conversational manner. I suggest you try this, since most people who read blogs expect the author to talk to be chatty and relaxed.
However, if you feel you must maintain a professional tone, then, by all means, choose that as the writing style of your blog. Only you know best what writing style will best communicate your message.
Well…your readers know best as well. You can ask them what they think or what they like. Try polling them. (You can add a poll to your blog easily.) If you aren’t blogging savvy enough to add a poll, simply ask them to give you feedback. At the end of a post ask: “Would you prefer that this blog be written in a more conversational and less formal tone?” See what types of comments you get back.
If you aren’t getting readers—which means you have no one to poll or to ask for comments, you can change your style and see what happens.