On Twitter, you can search out any topic, see the related posts, and then send a tweet to the person whose post you are reading. Is there a way to do something similar on Tumblr?
I don't think there is anything exactly similar for tumblr. It depends on what options are there on the blog.
If the blogger has added an ask option, you can contact them. just add /ask to the url of the blog.
or by selecting ask option on the blog.
If there is a tweet button on the posts, and if you know the twitter id of the blogger, you can tweet mentioning the blogger and the post.
check the contact option given by the blogger.
You can also comment on the post if available.