I had an issue with a blue cast on the corners and edges of my converted scans.
This vignetting issue turned out to be caused by LR lens correction. LR will apply a default value to correct lens vignetting for any particular lens that LR has a profile for. It does so by increasing the exposure in the corners and along the edges. While this is fine for normal photos when it’s applied to an image of a negative it has the opposite effect and actually increases the vignetting. You can see this this by increasing the exposure of a converted RAW negative in LR , the image goes darker.
The severity will depend on the amount of lens vignetting correction LR applies, my Tamron 35mm macro lens has a lot of auto lens correction applied so I had a blue cast in the corners and along the edges.
Neg Lab Pro appears to turn on lens correction, even if I disable it in LR, when I convert the image with Neg Lab Pro the lens correction box gets ticked.
Once Neg Lab Pro has converted the image and I’ve applied it, I adjust the vignetting in LR by moving the slider to the left until the blue cast disappears and there is no vignetting.
The blue cast is really just underexposure and you would expect it just to go darker but it goes blue.
I hope this post is of value to anyone with a similar issue.