In my case, they serve somewhat different purposes, though they do overlap a bit. Forums are a social thing, with the focus on sharing thoughts and ideas. I derive a lot of inspiration from the ideas and accomplishments of others, and I learn a lot about those things which fall outside of my own pursuits on the forums.
My blog is a combination of things. Mostly, I think it has become a tool for organizing my thoughts, and to help me keep focused on ongoing projects. My hobby time gets interrupted a lot, and a lot of times I pop onto my blog to get back in rhythm, after an interruption of days or weeks.
It is also easier to post photo heavy things that I want to share, and serves as an online place to store photos that I share elsewhere.