I logged into my Micro.blog account to look around, and it’s not at all clear how you would get a token specific to a blog. I only have one blog, so not sure I would be seeing the same thing. Probably worth asking over on the Micro.blog forum about the right way to get a Micropub token for a specific blog.
That said, once you do get the right token, if you are using this example action and want to use it for more than one different blog, it would require a minor modification to use a unique credential identifier for each blog you want to target…
// the "Micro.blog" in this line would have to be different
// in each copy of the action you want to target a different token
var credential = Credential.create("Micro.blog", "Insert Micro.blog app token generated on Micro.blog account page.");
Note the Send to Micro.blog example uses the Micro.blog app, which would, I presume, allow you to select which blog to use once it opens might also be an option.