It could be understood by 별 (which means ‘special’, ‘especially’) + 로 (which means ‘as’) as well. That is, basically 별로 means “especially” and typically it has meant “not much/not especially”. Why it contains negative meaning is that typically 별로 was combined with a negative predicate.
이 국수는 별로 맛 없다.
This noodle isn’t so delicious.