Forums encourage discussions. Blogs are personal to you and can be for anything you want tbh.
While you can make a blog post asking questions on something, I think you’ll be less likely to get an answer compared to just making a forum post on it. The only way a stranger might see your question on a blog is if they’re lucky enough to spot it in the flood of new blogs OR they actually take the time to look through your profile first. Recent forum threads are less likely to be as flooded and if a person’s question is answered, they can close the thread and leave more room for unanswered threads. Basically, if you want more sure way of getting a question answered, make it into a forum post.
Like I said before, blogs are also more personal. You can customize them, which is something you cannot do for forums. You can talk about your day in blogs. I...guess you could do that on the General forum, but the whole point of forums is to discuss stuff so if you’re gonna write paragraphs about your day, it should probably be eventful at the very least... Still, I’d recommend blogs for doing diary stuff :P