The primary purpose a photography blog would serve is to showcase your work. In fact you could easily turn your blog into a mini e-commerce site where people can buy your photographs. You could even partner with other photographers and put up their work on your blog for sale.
Recommended Read: A Blueprint To Making Money Online.
Apart from showcasing your work, you could cover the following areas on your photography blog [post ideas]:
1. Photography tips; segregated into tips for amateurs and professionals.
2. Technical guides.
3. Explanation of terms related to photography.
4. Useful software and apps.
5. Photo guides to tourist destinations.
6. Camera reviews.
7. Lens reviews.
8. Online tools for photographers.
9. Announcement of photography courses and workshops. [You might also announce your own photography workshop]
10. Initiate photography contests.
11. Buying guides. [cameras, lenses, accessories etc]
12. Photo editing.
13. Launch your own eBook on photography.
14. Specific guides on niche photography [Sports, Wildlife, Fashion, Child photography etc].
15. Featuring work of fellow photographers with appropriate credit.
Your blog could also feature a forum where photography enthusiasts can connect with each other.