It fails to “blend edge” along the intersection between the two objects (strangely the edge fillet command works, the blend edge no).
blendedge failed.3dm (956.7 KB)
It looks like the problems with BlendEdge are occurring where the edge curve segments are almost but not quite G2 (curvature) continuous. These are the result of two adjacent surfaces not being quite curvature continuous. BlendEdge creates blends which are curvature continuous between surfaces while the fillets created by FilletEdge are only tangent continuous with the surfaces.
I replaced the fillets between the flat surfaces which are tangent continuous with blend surfaces which are curvature continuous and the edge curve in those areas is now G2 continuous. BlendEdge now completes. However there are still some areas on the underside where the blends do not trim properly. These areas appear to be associated with other locations where the edge curve is not curvature continuous. You can either manually trim and join these blends or make the surfaces curvature continuous.
blendedge failed_DC1 small.3dm (1.4 MB)
An alternative to replacing any surfaces is to do the BlendEdge in segments of the edge which are curvature continuous, and then manually trim and join the results.