A nice touch is that the 2 pack lets you keep one relay in a tool kit.
Mini Micro PCB Panel Slide Switch Assorted Kit 2/3/4 Pin for Arduino DIY
You will find that the black color resists showing dust and smudges better than lighter options over time.
Because it is black, the relay looks uniform when you group several units together in a row.
These general purpose relays show up a simple black unit, which keeps the overall appearance tidy and consistent.
You will find that the CSJ-38-70010 behaves reliably in hobby projects and small control boxes.
The black color gives it a neutral presence that doesn't fight with your wire colors or labels.
You will find that the black finish helps keep the setup looking tidy even in busy cabinets.
The black color is a nice touch because it helps the relay blend into dark enclosures and backplanes.