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.

I want to add some more examples to Hong Minhee's answer.^___^ 1) 나는 옷차림에 별로 신경안쓴다. I don't care what I look like. 2) 몸이 별로 좋지않다. I don't feel well. 3) 난 그런것 별로 좋아하지 않는다. That's not my cup of tea. 4) 지금은 별로 먹고 싶은 생각이 없어요 I am not in the mood to eat right now. 5) 별로 마음이 내키지 않는다. I don't feel like doing this. 6) 요즘 기분이 별로 안좋아. I have been feeling down recently. 7) 음..글쎄 별로 인거 같은데 Well, it was nothing special. 8) 별로 볼만한게 없다. There is no great to see. 9) 어제는 별로 한일이 없어. I didn't do anything much yesterday. 10) 어쩐지! (별로 놀라운것이 없을때) No wonder!!! ^________^