I have not found flash rust to be a problem, but then I wash the parts and oil them. The black stuff seems to be a mix of treacle and converted rust products. Most washes off well, and I don't mind the patina that remains. I will see if I can get some photos
Caution though - if you forget about the part and leave it in for many weeks you may find it totally black, porous and thin! It is slow acting but still needs watching. I would say this is comparable to electrolysis and acid in that regard.
Best bet is to try it on a rusty nail, or bolt, or neglected file, and see what you think for your application
I have used black treacle from the supermarket with good results, but if doing large parts I would go to a farming or equestrian feed supplier as they sell molasses for animal fee in bulk
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