The method name must start with either findBy or findOneBy. Undefined method Symfony?

Make sure that you have modified your entity class:

// src/Blogger/BlogBundle/Entity/Blog.php
/**
 * @ORM\Entity(repositoryClass="Blogger\BlogBundle\Repository\BlogRepository")
 * @ORM\Table(name="blog")
 * @ORM\HasLifecycleCallbacks()
 */
class Blog
{
    // ..
}

the annotation @ORM\Entity(repositoryClass="Blogger\BlogBundle\Repository\BlogRepository") is required.

And don't forget to regenerate entities:

php app/console doctrine:generate:entities Blogger

UPDATE

Remove annotation @ORM\Entity. It overrides correct annotation @ORM\Entity(repositoryClass="Blogger\BlogBundle\Repository\BlogRepository")

In my case adding proper annotation was insufficient.
Deleting Doctrine Cache by php app/console doctrine:cache:clear-metadata also not worked.

I generate my entities from database by commands

php app/console doctrine:mapping:import --force AcmeBlogBundle xml
php app/console doctrine:mapping:convert annotation ./src
php app/console doctrine:generate:entities AcmeBlogBundle

First command generate orm.xml file for each DB Table in my project. After DELETING all orm.xml files Annotations started work properly.