1. A method for performing blends in a geometric model, the method performed by a data processing system and comprising: through operation of at least one processor of the data processing system: receiving a geometric model including a plurality of blends, wherein each blend corresponds to a geometric curve or a face adjacent and between at least two unders that correspond to geometric faces or curves, wherein each blend blends the at least two unders together and which has a shape that is dependent on the positions of the at least two unders; identifying a blend ribbon, wherein a blend ribbon corresponds to a set of at least two of the plurality of blends, wherein each blend in the blend ribbon is mutually dependent on at least one other blend in the blend ribbon such that the mutually dependent blends are unders of each other; receiving through an input device at least one input representative of a manipulation of at least one of a blend or an under; responsive to the at least one input, selecting at least one blend ribbon breaker for the blend ribbon, which blend ribbon breaker corresponds to the manipulated at least one blend or under and optionally at least one other non-adjacent blend or under; breaking circular dependencies of mutually dependent blends by designating a plurality of non-adjacent blends in the blend ribbon as reblendable blends and designating at least one of the blends in the blend ribbon as non-reblendable, based on the selected at least one blend ribbon breaker; modifying the geometric model to produce a modified geometric model based on the at least one input including reblending the designated reblendable blends to have a modified shape while not reblending the designated non-reblendable blends; and storing the modified geometric model.