Why are there visible artifacts in my imported geometry?

When I import this mesh into Unity I get ugly visible artifacts splotches on the surface:

Here I use a matte red-ish material and the black and grey crud is clearly visible. What is happening here? How do I fix this?

I calc normals and tangents in mesh import.

I used Blender to inspect the data and look for things - inverted normals, etc. - to clean up but saw no issues.

Also, this model renders fine in an iOS/Metal app I wrote using Xcode.