Reblogging may be the standard way of responding to posts on Tumblr, but it's not the only one. Depending on how the blogs you follow are configured, you may also be able to leave comments on posts directly from within the dashboard. For this option to be available, the blog owner must have enabled replies from people she follows, people who have been following that blog for more than two weeks, or both. Neither option is on by default, and the first is available only for primary blogs.
Occasionally, you may also see Question posts on your dashboard -- posts displaying a comment field. This is an option that can be enabled when creating posts that end in a question mark; you can answer them regardless of whether you follow that blog or not.
Leaving comments and answering Question posts can't be done anonymously or privately. The comments and answers you leave are displayed among the notes to the post, both on the dashboard and on the post's individual page; your primary blog's username is associated with each comment. If you want to contact someone privately, use Tumblr's messaging features.
Leaving a Comment
Click the speech bubble.
Type your comment in the text field. The comment can be at most 250 characters long.
Answering a Question post
Click anywhere inside the comment field to activate it.
Type your answer in the text field. The answer can be at most 250 characters long.
You can also answer Question posts from that blog's main page, or the post's individual page.