refinerycms-core requires Rails so don't check if defined?(Rails).

lib/refinery/blog.rb

@@ -6,8 +6,7 @@ module Refinery
   autoload :BlogGenerator, 'generators/refinery/blog/blog_generator'
 
   module Blog
-
-    require 'refinery/blog/engine' if defined?(Rails)
+    require 'refinery/blog/engine'
 
     autoload :Version, 'refinery/blog/version'
     autoload :Tab, 'refinery/blog/tabs'

lib/refinery/blog/engine.rb

@@ -1,5 +1,3 @@
-require 'refinerycms-blog'
-
 module Refinery
   module Blog
     class Engine < Rails::Engine