To create a blend curve constraint to an isoperimetric line, snap the blend points onto the isoperimetric lines. The resulting blend curve has a GEOMETRIC Curvature type. Because of this calculation method, the blend curve is not following the isoperimetric line.
To follow the isoperimetric line, select the blend points first then change the Curvature type to PARAMETRIC. The shape of the curve changes so that the blend curve properly aligns to the isoperimetric line.