It is very simple to enable comments for blog pages.First

Goto Posting-->Edit Pages

Then the pages in your blog will be displayed.From that choose the page for what you want to enable comments,there will be three option for each page edit,delete and view in that click edit then the page will be open,in that page see below

Post Options will be there click that

Reader Comments option will be there with three options

In that three options select allow comments if you want to enable reader comments for that page.
default option is don't allow,hide existing.