Official Pinyin of 《卜》 in 《卜姓》： "Bǒ", "Bo" or "Bǔ"?
This family name is very rare. I personally don't know anyone around me with this family name. The authoritative dictionary "新华字典" lists Bǔ as the correct pronunciation for the family name. 

The pronunciation "Bo" (neutral tone) is only used in the word 萝卜, which is a plant (radish), and we even have 胡萝卜(carrot).