What is MBL? MBL stands for Mid Back Length. This is yet another reference to hair length in which the hair extends down to the midpoint in the back. POSTS RELATED TO THIS TERM Terminal Length – How Long Can Your Hair Actually Grow? How To Get 4b Hair To Waist Length In 3 Years 10 Reasons You Are At A Hair Length Plateau – Black Hair Information TweetPinShare0 Shares Home » Glossary » M » What is MBL?