这里的being blind 充当什么成分？

有一读者问： 高一英语课本中有这样一句话：

They had often heard of elephants, but they had never seen one, for, being blind, how could they?

课本有注释，但语焉不详。being blind前后为什么有逗号？ 它到底充当什么成分？

being blind前后的逗号皆表停顿，起了使文字醒目的作用。它是一现在分词短语，其成分是原因状语。