I've never done this myself but I would expect that there's some "break in" wear such that there is an initial small but fairly rapid amount of wear that leads to some "stretch" followed by a long period of gradual wear until the chain is considered worn beyond further use. If this is the case then some increase in the production of the black stuff on a new chain would be expected.

I've never noticed this tendency since the conditions I ride in are far from labratory clean and never really thought much about it.

The wear in the chain takes place at two significant interfaces. First is the pin to bushing wear between the pins and the swaged bushings of the narrow links that fit inside the wider outer links. Second is the wear that occurs between the rollers and the swaged outer surface of those same inner link plates. But the only part we measure is the inner joint wear when we measure the chain. Yet the wear to the rollers also plays a part in how the chain will fit the sprocket teeth.

When hanging between the sprockets in the free area the chain connections are not under any significant amount of movement since the chain is purely in tension. So metal to metal contact should not cause any wear. Although I guess it could cause some degree of pressure displacement in a new chain with smaller contact points if the pressure at some small contact should rise above the plastic flow point. The rotational wear in the chain only really takes place at two points. Where the chain passes off the rear sprocket and where it rolls onto the front chainring. And in reducing values for some number of sprocket teeth around the circumference of the chain since it's not just one tooth of the sprocket that is engaging the rollers. But it's fair to say that by the time you look at the links and rollers about 1/4 to 1/3 of the way around the half circle of chain engagement that the links won't actually be transferring any load to the teeth. The load being taken up by the new teeth coming out of and into engagement at the top of the sprockets. And since the lower chain run isn't under any tension other than the spring in the derailleur any wear at those points of rotation would be minimal and caused only by the abrasive value of the oil and road grit that exists at the time.

Yep, all in all it's a wonder we can pedal our butts down the road with such an exposed and mistreated drive train concept....