We did see exactly this (dewetting on PTH pads leaving black nickel exposed) 
on some batches of product from one supplier back in 2006.

XRF analysis showed that gold deposit was thin (borderline acceptable) and by 
microsection of failed joint that the underlying nickel surface topography was 
very rough, due to excessive mechanical abrasion of the copper prior to ENIG 
deposition. Theory was that peaks of the rough nickel were poking through 
thin gold especially at the edges of PTH pads.

We don't use this supplier anymore...

Hope this might help

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