I've had several queries about charging for blogging. "How much should I charge?" is always a thorny issue for new copywriters. :-)
Your fees depend on the number of blog posts per day, the length of the posts, and the amount of research you'll need to do for each.
I find that an average blog post of around 150 to 300 words takes me around half an hour when I know the topic, so I charge half my hourly rate.
If the blogging job is on contract over several months, you can lower the rate per post, because you'll be able to research several items at a time, so the writing goes more quickly.
However, do beware of getting into a situation where the blogging takes up a lot of working time each day that you're not being compensated for because you've charged a "bulk" rate.
Blogging can seem like a good gig because you've got a job, so to speak, that extends over months. But several colleagues have told me that these jobs take so much energy that they've given up blogging for others because it cuts their hourly rate right down.
My recommendation: trial the gig for a couple of weeks (with pay), and keep a time log. This will help you to set a realistic rate for a blogging series of three or more months.