blog.rb

class Blog
  attr_writer :post_maker

  def new_post(options = {})
    post_maker.new(options).tap do |p|
      p.blog = self
    end
  end

  private

  def post_maker
    @post_maker ||= Post
  end
end

blog_spec.rb

describe Blog do
  let(:blog) { }
  let(:post) { }

  before do
    blog.post_maker = double(:new => post)
  end

  # ...
end