For me kisslicer is a good indicator tool to find out where a stl is
degenerate. This is what it indicates with your STL:

http://forum.openscad.org/file/n17460/Puzzle.png

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So, what does a 'blade edge' mean?

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"blade edge sharp crease" is what it says and mostly results from numerical
errors. (Almost) degenerated triangles will most probably not show up as
'desired' in the sliced path. A closer look at the triangulation (CTRL+1)
unveals what has happend.

Kisslicer indicates those errors, but silently gets around most of them.
Redness of colors indicate severity of errors.
http://forum.openscad.org/file/n17463/puzzle1.png

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