Tell, Don't Ask when dealing with displaying model attributes or a null replacement
Definitely, this does not belong in the model. The orthodox way is to create a helper. Note how you can use the pattern Object#presence:

module ApplicationHelper
  def show_field(field, value)
    value.presence || "#{field}: unknown"
  end
end


And use it in the views:

&lt;%= show_field("Name", user.full_name) %&gt;


You can go a step further and use OOP view presenters if you like the idea (personally I don't use them).