Messages are pretty simple, but if you want comments, you have to utilize a 3rd party site (such as Disqus) to allow comments. Check Tumblr help. Best of luck!
This, however, was a “question and answer.” I posed a question in a blog, which Tumblr recognized. This allows and option entitled something like “Would You Like People To Answer?” Click it. Sorry if that’s not too specific, I.T. is not my strong set!