How to write a commercial blog without being an expert?

Maybe you don't

I don't know how you would write an active blog about a topic you do not know about. If it is a one time thing, maybe you could fake it but an on-going blog would be tough to keep up without getting exposed. You might have to have the client sketch out blog pieces and/or details for you to put into a decent form and style but to write what you don't know at all... Might not be worth it. Could backfire on your reputation.

I would say it depends: How good are you at research, how much are they paying (can you still make a decent rate when extra research time is considered), and how badly do you need to break into this niche?

I've written corporate blogs on a variety of subjects I'm not an expert in, most notably a construction software blog. The thing is, I made sure the client was aware I'm not a subject expert, but could demonstrate research ability that would help balance out my lack of off-the-top-of-my-head knowledge.

If you decide to take the gig, be sure to clearly lay out who's going to come up with post topics and get the topics reviewed by your client before writing them. That ensures you're going in a direction they want for the blog, and helps narrow down your research -- because you're right, "internet marketing" is way too broad. And when you don't have a lot of subject expertise, you don't always know the best way to narrow that subject to blog post topics.

Internet marketing is on of those fields where the blog really matters. It is part of the internet marketing for the company. If it doesn't show well, the whole company would like like it can't do the job.