We released version 1.5 of the Micro.blog app for iOS today. In addition to a couple bug fixes, it lets you pick a category for new posts. You can create categories on the web under Posts → Categories for blogs hosted on Micro.blog.
Tap the gear icon when composing a new post to view your categories:
Manton Reece @pratik It's for any length of post, but the idea for now is that the category options are hidden when writing short posts on the web because I don't want to complicate the posting UI.
Manton Reece @pratik On the web, yes, first the title field appears, and then as soon as you type something in, the categories appear. We may tweak this. On iOS/macOS, you can pick the categories at any time so they don't automatically appear.