Thanks for this PR!

Although there is no longer a crash as described in #690, the blend doesn't result in correct geometry, which is visible in DRAW and in FreeCAD:

Below a transcript in DRAW with some checks where Face #3 is the highlighted face in the screenshot:

$ DRAWEXE 
Draw[1]> Draw[2]> source curved-base-sweep-curve-opposite-dir.tcl
tolerance ang : 0.01
tolerance 3d  : 0.0001
tolerance 2d  : 1.0000000000000001e-05
fleche        : 0.001
tolblend 0.01 0.0001 1.0000000000000001e-05 0.001

Draw[3]> vdisplay result
Draw[4]> vfit
Draw[5]> vselmode 4 on
Draw[6]> vstate
Locally selected sub-shapes within result:
       Face #3

Draw[7]> checkshape result
On Shape faulty_1 :
BRepCheck_UnorientableShape
Shape faulty_2  on shape faulty_1 :
BRepCheck_NoCurveOnSurface
Shape faulty_3  on shape faulty_1 :
BRepCheck_NoCurveOnSurface

Faulty shapes in variables faulty_1 to faulty_3 

Possibly the other related issues give some more handles to investigate.

Yes, the output is not valid. The issue were related to crash.
And I saw your one more ticket related to the quality. It will resolve later.
As an option I would recommend to report new ticket with invalid result. Originally it is our part to re-create issue with new description, but you already created one with quality. Probably you can extend existed

Totally understand. The PR fixes the crash, so the original issue can be closed. Sure, I've already investigated, so it's easy for me to create a new issue for the quality.