last summer i paid like ten dollars for a bottle, put it in a marker, and made marks, mainly on silver electrical boxes.
a week later i came back to take a look at them, and the tags were all but gone. faded to the point that you'd have to be withing about 10 feet to see the ghost.
even worse, the ink spread out past the path of the original stroke of the marker, and obscured the tag into illegibility.
also, the color isn't so much black, as a very dark violet/blue, and it's not very opaque. if you have to use it, mix it like 1-2:2-3 with pilot, marsh, otr, or some other opaque black ink.
the only decent experiences i've had with nero are insides, where they stained nicely.
to summarize, don't use it outside unmixed.
postscript; how is black garvey? is it opaque? does it fade as easily as violet? and is marsh alcohole or xylene based?