The Blue and Black straps are identical, other than the color there is no difference both are medium to double extra large size. They supply you with 2 straps in case you damage one you have a spare while you order another. I use the blue strap and let my wife use the black strap when we go to the track, I will do my sprints and when i am done, I pass the X2 device to her strap and then she does her sprints.