What is a blog? Well, you know what one is. You read blogs often - or from to time to time anyway. But you may want a definition all the same. So I'll point you to a few. Here is Wikipedia's:
A blog (a contraction of the term "web log") is a type of website, usually maintained by an individual with regular entries of commentary, descriptions of events, or other material such as graphics or video. Entries are commonly displayed in reverse-chronological order...
Many blogs provide commentary or news on a particular subject; others function as more personal online diaries. A typical blog combines text, images, and links to other blogs, Web pages, and other media related to its topic.
Here are some other definitions, all much of a muchness. Note that the Wikipedia entry also has this:
The ability for readers to leave comments in an interactive format is an important part of many blogs.
And according to this, it is most blogs that enable comments. So, some blogs don't enable comments though many or most do. Along with the definitions sampled that are silent on this score, it would appear to be the case that blogs either do allow comments or they don't.
However, you will occasionally come across the view that if a site doesn't allow comments it isn't a blog, and this view was expressed with some feeling yesterday by Tom Paine at The Last Ditch: normblog, he insists, is 'never a blog... [because] a blog provides the opportunity for readers to comment'. Be my guest, Tom. If you want that normblog isn't a blog, feel free to call it something else - a lobg, or a glob; or a blocg, maybe. I'm not that bothered. People can name things how they want. But here's another way of looking at the thing. Along with other... er, websites that don't enable comments, normblog is commonly referred to as a blog, and you shouldn't take your own preference for all of reality, even it is only semantic reality. Tom wants to be able to comment on something I write? He can do so. He can do it by email to me, or he can do it for anyone else's benefit on his own blog. He has a perfect liberty there. But should he want just to have his comments on my blo[c]g, then tough - unless it is by arrangement. Long live libertarianism. I have given my reasons here. Tom isn't even obliged to agree with them. It's a wonderful world, full of variety. And part of that variety is that Tom isn't the sole arbiter of public meanings.