As far as I understand your question, the unsubscribe url will be in the contents of the email.
For getting that to work, I'd suggest you to generate a unique identifier to the mail template which you can then use to identify the user that is unsubscribing.
Check: http://guides.rubyonrails.org/action_mailer_basics.html point 2.1.3 - Create a Mailer View.
Lets say that your template would have something along the lines:
<p>
Here is your email contents
</p>
<p><a href="#{@unsubscribe_url}">Unsubscribe</a></p>
This unsubscribe_url variable would generate you something like:
http://your-domain.com/newsletter/unsubscribe/unique-id-generated-for-the-user
Create a route in your config/routes.rb file that takes you to the controller that handles the unsubscription.
You can also take a look at here: http://railscasts.com/episodes/312-sending-html-email
EDIT
Unfortunately, from what I see in Mandrill's documentation I don't think you have a way to do a 1 to 1 mapping of the merge variables with the emails in the bcc list.
This way I see two possible solutions:
a) You don't really have a unique id for each user but rather you have a generic link that will take you to an unsubscribe page.
For example:
http://your-domain.com/newsletter/unsubscribe
This would render a page with an input where you'd ask the person to write their email address.
Using this solution, you'd have to take in account bad user behaviour such as:
- inserted email does not belong to your user list
- alice typed bob's email
b) you break the logic of email requests to mandrill. This way you'd keep the unique id login but instead of sending a list of emails to be in bcc, you'd send email request one by one.