I am now rolling my own blog app and I am kind of newbie to Rails 3. Even like that, in 1 week i have a blog with tags, comments, authentication with omniauth, etc.. my advise is: roll your own. I was having the same doubt and looking for pre-made solutions and I decided to start it from zero and just look for plugins for anything i need.
It goes pretty fast if you know already some rails programming and you use the right plugins. This is what i used:
Omniauth to let users be able to autenticate with facebook, twitter etc.. and leave you comments.
rails_admin: it allows you to manage your blog by going to yourapp.com/admin. It uses devise to create an Admin user (you can specify a diferent model name than user to not to mix it with the users from omniauth or from your other app) and if you have the right models and associations between them you can from there create your posts, assign them tags or categories and also delete comments etc.. its all done in an easy way. For the Text Area that you use to introduce the content of your posts you can associate it with the ckeditor just by adding to the rails_admin initializer something like:
config.model Post do
edit do
field :body, :text do
ckeditor true
end
end
end
And with the ckeditor you can introduce pictures, attach videos, format text, and so on.
Use kaminari for pagination, or you can use will_paginate if you are more used to that.
Using the blueprint framework for styling with css you will save time and have a more standar styling.
Use few jquery lines to insert/delete comments graciously.
And that's all I can remember now. And if it shouldn't interfere with the main app, i would just assign a subdomain for it. So if you go to blog.myapp.com you access to the blog and if you go to myapp.com you access to the app. And you want users from the app to interact with the blog so you should use just one app and have this 2 subdomains pointing at differents parts of the same a app.. take a look at: rails 3 - one app, multiple domains, how implement a different 'root' route for one of the domains?
That's all i can think now! let me know if i can help you in anything else.
