An academic or professional blog would be especially useful in which of the following fields?

Blogs, even academic and professional, are not usually considered reliable sources. Most importantly, that's because they don't undergo the peer review process, which ensures the quality of the published papers (that they don't publish lies, in simple words).

So usually, you would prefer a published paper.

When then would you use a blog?

One such instance is when you want to find out about a quickly changing field, for example the newest standards in software development. So software engineering is one such field.

Another field is internet protection and security, as the viruses and the ways to defend against them also develop faster than can be published in a paper