Old UV transmitting lenses need an UV focusing indicator, like the well know IR red spot.

So the "blue spot" turned out to be at the (closer focus) f8-f11 indicator of the DOF scale
for this 100mm triplet lens (this cannot be generalized). All shots were done at f5.6
(Rows 1 - 3)

A second test with the same lens design, but a 50mm lens confirmed this finding.

A third test with a different lens design and 135mm focal length showed nearly no focus shift at all (Row 4, Macro test Row 8)

Reference Shots:
1) Coastal Optics Micro Apo UV 105mm (Row 5) @f11
2) Nikon UV Nikkor 105mm (Row 6) @f11
3) Details for comparison (Row 7, 8, 9)