My Blog section contains all posts that are reblogged by you. It can be your post or other's post as well. On the other hand, My Post section contains your own posts only.

It seems that if I post something to my Blog, it goes to an obscure hole that no one else would see unless they specifically, for some unknown reason, come and look at my profile.

This is a misconception. Your post isn’t going anywhere. If you publish a post in your blog (not in any community), then there's a good chance your followers might visit your post. Exactly, that is why you should build/grow own audience on Hive. Also, as an alternative, you can publish your contents in popular communities where there are enough audience to go through your contents.