1-It doesn't matter what your business is, your first concern is always your customers; what do they want to hear about? Of course identifying that is easier said than done because people don't always know what they want. A lot depends on the industry you are in but observing customer trends is a sure way to find suitable blog subjects. Your customers are the target audience for your blog just as they are the target market for your product; if your blog appeals to them, even if the subject is not specifically to do with your business, then your company will appeal to customers by association. See what gets the best reaction, what attracts comment or discussion, and focus in on that.
2-Play to your strengths both as a writer and as a company. If you are not making at
least irregular reference to your company's USP (unique selling point) then you are not making the best of your blog. This does not mean that you spend paragraph after paragraph extolling the virtues of specific aspects of your company, a blog is not an advert and those that try to be can be very off-putting for the casual reader. You need to take a more subtle approach; tell a story in which the strengths of your company are part of the story but not the point of the story. For example; I am seeking a publisher for a non-fiction book, I do not write blogs that name check the book, but I write on the same subject in a way that demonstrates my knowledge of that subject.
3-Be aware of your competitors blogs. Do not try to copy them or to answer them in a
direct way, if you do you run the risk of getting into a blog-based argument and your customers will soon lose interest in that. What you should do is respond to them in an indirect way; choose deliberately contrasting subject matter, or address the same subject but in a way that is more typical of your company. Your competitor is likely to be highlighting their company's USP, so don't try to take them on, focus on what you are good at but be aware of what they are saying so that you do not look bad in comparison. For example; if your competitor makes an offer and then you make a similar but worse offer, you will look bad. Being aware of your competitors will avoid this.
4-Don't isolate your company from the rest of the world. Your customers' lives are
affected by factors other than your company so be aware of that. Your blogs should be seasonal and topical, addressing things that your customers are facing at the present. Choose a subject that is seasonal or topical and bring your company into it. This will make the blogs more readable and less predictable, but will also bring your company into the lives of your customers. Regardless of what your company is Christmas will have an effect on its customers, changes in economic policy will affect them. You don't have to make these situations less stressful or less financially taxing for your customers, just identify with them, operate in the same world.
5-You're writing a company blog but that is no reason for it to be dry and uninteresting. People need a reason to read a blog, it gives nothing back to them, so make it entertaining, give them a reason. You don't have to make stuff up, tell stories or be hilariously funny, but write as if you are talking to a friend, not addressing a board meeting. Your subject choice can reflect this too; tie your company to a subject that is intrinsically interesting. As always this depends on what your company is and it's important that you don't overdo it; don't pick a subject like lion-taming and try to link it to your company, start from your company and work out; what interesting subject can you legitimately link it to?