Hi @Anonymous
with this form, those profiles and the spline curve, it would be too hard to impossible to achieve that using a swept blend...the geometry will always break, intersect itself ...etc; causing the error stating that Revit cannot create the requested geometry.
If youdo not want to go into a a more complex method, you can try to simplify the profiles, change dimensions (ex: too thin in such case is no good), add the missing step in the 2nd profile... see if it works (although I doubt at least not to your liking)
Personally I would model that in conceptual massing environment and I would be using an adaptive profile with more than one path to track the sweep. @constantin.stroescu have posted a screencast to one thread on a similar topic and a similar method (but without adaptive profile)...You can check it out on the following link
http://forums.autodesk.com/t5/revit-architecture/how-to-model-organic-froms-in-revit/m-p/6039107#M109116
Let us know if this helps
