How to create a Question/Answer block?
If you just want to create a node where a user writes a question to which the other users answer using comments, then you can use a content type that Drupal has by default. For example, with Drupal 7 the "Article" content type has already what you need; an article appears as in the following screenshot:



The only things that would need to be changed are the subject of the comment, that should not be entered, and the string "Add new comment" that should be changed in "Add new answer."
For the first, you need just to change the settings for the content type.



If you deselect the highlighted checkbox, the user will not be able to enter a title for the comment.
The other option to which you could be interested is the following one:



When it is selected, any comment for a comment is shown indented. As the comments for the node are the answers given to the question, any comment for the answer should be indented, or it would not be possible to understand when a comment is an answer, or a comment for an answer (or a comment to a comment given to an answer).

For changing the string from "Add a new comment" to "Add a new answer," you need a custom module that changes that string. For this reason, I would create a content type copying the setting used for the "Article" content type.

You don't need to alter the page.tpl.php file to show the comments together the node: That is what Drupal normally does. There is a module that allows to shows the comments in a different tab, but without that module, Drupal shows the comment in the same page where a node is shown. You will eventually need to change a template file to change the layout of the comments, though.

If you are using the word block to mean Drupal block (e.g. you want to show a block for asking a new question), then that is not possible without using third-party modules. I am not sure there is already a module for that.