Space Black Manufacturing Process Imperfections exist but are "normal".
Out of all the Space Black units I have seen in store, exchanged - I would say 80-90% of them have some machined type imperfections. The machined edges on a lot of the Space Black models can show the silver aluminum on the edge where it is cut/sculpted (see pictures attached). I have seen this around the trackpad, keyboard well, rear hinges, side vents, display lid and bottom panel edges. Some are nearly imperceptible and some are much more noticeable.
These imperfections are on all colors, but are most noticeable on the darker space black color. I spoke to an Apple Store manager and he said he had seen this on some of the in store models and it was just the nature of the manufacturing process.
I have personally exchanged a few models due to some more noticeable imperfections and the latest model I have is the cleanest I have seen. It has a slight exposed silver edge near the rear left hinge and a very slight silver exposed edge on the bottom right front, almost imperceptible. As I have seen this on almost all Space Black models, I have come to terms with it being "normal" for the color. Only other choice is to go with silver which my OCD definitely has me debating, but the Space Black does look amazing.
On a side note, one of the models I had exchanged showed a chip around the USB-C port after being used only once (see image attached), so I would recommend using the MagSafe port for charging or if you do use the USB-C port, get a magnetic adaptor like
this to reduce the amount of times you are attaching and removing the cable.
Space Black definitely is more susceptible to showing wear so be very careful or just go with the silver color.