If you have a look at the Doctrine\Common\DataFixtures\AbstractFixture
class and view the ->getReference()
function it will show that variable passed in for the function is for a string.
This means that the first example, calling getReference
twice is the way it will work.
$blog1->setCategory($manager->merge($this->getReference('category-1')));
$blog1->setCategory($manager->merge($this->getReference('category-2')));