I've tried all the various types of nuts before and my personal favourite is bone as it seems to give the most natural response for an electric guitar. For an acoustic guitar , I tend to favour tusq from graptech.
As for brass, it gives a certain metallic ring to the open strings which may be good or bad. There's also a tendency for strings to bind in the brass nut if the nut is not slotted properly.
As for the black graphtech nuts, if you use a tremolo system of some sort its always a good idea, but no reason for a well cut bone nut or any nut to give tuning problems.
Hope this helps
KC