They passed on him because of size, fumbles, and lack of production in college. But you have to remember, Blue played at Texas recently where they will showcase multiple backs most of the time.
Blue's lack of production actually surprised me because Blue made a lot of big time plays, so I just assumed he got more reps. He just stands out as someone who made it look easy, and I think his skillet translates well to the NFL. I'm sure if teams weren't drafting him, they also had scouts throwing him out there as an option.