I think as far is Minimins is concerned the difference is a diary here is a thread, which other people can also post on. A blog is a diary, but other people comment on rather than reply to a post. This then means it reads more like a diary, as all the entries are from the blogger, you need to click on a separate comments button, to read other peoples views.
Hope I have explained it correctly, it seems really complicated now I have written it down.