Well, there are different ways you could go about it. You could add it as a part to blogs and check what tags the blog has and display other blog items with the same tags, then use placement to put it wherever you want. That might be a possible way to go
Follow this guide to learn more about building parts and stuff in Orchard, if you are not sure.
And then check in the Blogs module to see how they find the most recent blogs and display them. Steal that code and just change it to find blogs with similar tags.
Alternatively, you could make a widget, although I'm not sure how that would work since it wouldn't know about the blog post... I guess you could check the url when it loads, then load some data from the server based on the contentItem that url is linked
to. Although I don't know if that is possible. A challenge :)
Hope that helps a bit. If it doesn't, I'm sorry.