Hi niko, if I turn on surface control points of that middle piece, I can see that the area that you're having problems with is a collapsed down "singularity" of the surface:




It's quite easy for such collapsed down areas to have pinches in the actual surface shape, sometimes the pinching can be in a pretty small area too. That kind of irrregularity or bumps or little lumps in the surface shape will tend to have a noticeable affect on blending, since blend is trying to adapt itself to the surface shape.

If you look at the isoparms in Rhino, the middle isoparm seems to bend off towards one side right near its end, there is probably some undesirable sharp change in surface curvature right in that area.

- Michael