It is possible that you can find a small crack (approx 20 mm) on the aft edge of a blended winglet, around the area where the curved trailing edge meets the straight trailing edge. Do not worry, it is cracked sealant (and paint) which was applied to smoothen the transition between the curved and straight trailing edge structure. The sealant cracks because of the loads on the winglet. The crack can occur within several months after installation. See pictures below for more clarity.