It's ironic. We went to four (or five, or ten) years of college to learn, in part, how to write correctly and now that we're ostensibly professionals, employers are begging for the raw, candid power of a blogging voice. But before you toss that copy of AP Style in the trash, rip out the page about Oxford commas and burn it.
Just kidding. Keep it, because you're going to need it.
The writing style we have come to associate with blogging seems very ad-hock. People read a blog and they see a peer. They may come to think that they could blog. Which is why there are so many blogs on the Internet. It's also why there are so many bad blogs on the Internet. A good blogging voice can't come from just anywhere.
Hank Green of the Vlogbrothers spoke recently about what makes a successful video blog. Several of his points translate to text blogging as well.
You want your blogging voice to fall below over-produced professional-sounding language that will either come off as dishonest or boring, but somewhere above your average Aunt Jane's annual Yuletide Newsletter. Avoid overly academic, dry, corporate language. Use plain words, but be careful not to condescend. You want to emulate casual speech but you should steer clear of the sentence fragments and protracted contractions that don't sound weird when spoken, but will look completely messy in writing.
You don't have to follow AP style to the letter, my joke about oxford commas from before is not without merit, but you do have to pick a style and stick with it. You also have to proof read. And re-read. And re-re-read. If you write your own content and you don't have an editor, you have to be especially careful. Without a second pair of eyes, your own mistakes can easily slip by you.
Sometimes a firm will provide a copy editing service at a reasonable price. My firm, for example, has a copy editing rate of $0.10 per word of original copy with 2 free revisions.
The most important thing you want to do when cultivating a blogging voice is practice, practice, practice. Successful online marketing comes from a place of honesty, and if you don't have a genuine voice you're not going to sound as honest as you could. People may start reading your blog because you offer a solution to a specific problem, but popular bloggers will say that their readers stay for the story. How you tell that story depends on your personal writing style, and that can only be achieved through years of constant practice and refinement.