The format validator uses :invalid key by default, not :blank.

https://github.com/thoughtbot/shoulda/issues/#issue/119 by adrpac

lib/shoulda/matchers/active_record/validate_format_of_matcher.rb

@@ -49,7 +49,7 @@ module Shoulda # :nodoc:
 
         def matches?(subject)
           super(subject)
-          @expected_message ||= :blank
+          @expected_message ||= :invalid
           return disallows_value_of(@value_to_fail, @expected_message) if @value_to_fail
           allows_value_of(@value_to_pass, @expected_message) if @value_to_pass
         end

spec/shoulda/active_record/validate_format_of_matcher_spec.rb

@@ -15,15 +15,15 @@ describe Shoulda::Matchers::ActiveRecord::ValidateFormatOfMatcher do
     end
 
     it "should not be valid with alpha in zip" do
-      @model.should_not validate_format_of(:attr).not_with('1234a')
+      @model.should validate_format_of(:attr).not_with('1234a')
     end
 
     it "should not be valid with to few digits" do
-      @model.should_not validate_format_of(:attr).not_with('1234')
+      @model.should validate_format_of(:attr).not_with('1234')
     end
 
     it "should not be valid with to many digits" do
-      @model.should_not validate_format_of(:attr).not_with('123456')
+      @model.should validate_format_of(:attr).not_with('123456')
     end
 
     it "should raise error if you try to call both with and not_with" do