If you read IPC-D-275(IPC-2221) it recommends black oxide under solder mask
for large metallic areas.  I do not know anyone who is applying black oxide
tro improve solder mask adhesion. Some used pumice (MAY CAUSE DWETTING),
others microetch and some just clean. 
But since it is in IPC-D-275, some must be using it.

Phil Hinton 
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