Often when a business thinks about what type of content they should be writing for their blog, they tend to lean more towards prospective customers and generating leads. There's nothing wrong with that. More leads hopefully means more customers, thus more revenue. It makes sense. But solely focusing on new leads neglects an important group of people...YOUR CURRENT CUSTOMERS. You still have love for them right? Show it. Focus some of your blog content on them.
If you create content that your customer's enjoy and find helpful, they'll be more likely to share that content with colleagues than someone that's never done business with you. If you delight your customers they will turn into raving fans.
If you are truly providing value through your content the people reading it will realize you care. This is just another way to strengthen your business relationship.
When writing a blog you shouldn't be overly salesy, especially if you're looking for ways to provide value to current customers. That doesn't mean you can't educate them to the point that they are knocking on your door looking to buy again, buy add-ons, other services, etc.
Some of you might like phone calls from current customers and that's ok. But if they are constantly calling you and asking questions, it's likely they couldn't find the answer on their own first. If they are aware of a location that they can find answers to common questions about whatever they purchase from you, they'll probably look there before picking up the phone. Alternatively in a phone call you can refer your customers to the blog to find more detailed information about the answers you gave over the phone.