What stands out about the black color is that it suits a wide range of enclosure styles without drawing focus.
10pcs 2.54mm 40Pin Male Breakable Pin Header Strip 1x40P Single Row Black
With the black color, the relay blends in well, which can make the whole panel look more cohesive.
You will find that this relay carries the Deltrol brand, which I think is reassuring.
A nice touch is the quiet coil action, which pairs well with Industrial Electrical Controls & Automation > Electrical Switches.
You will find that the black color resists showing dust and smudges better than lighter options over time.
You will find that labeling on the BU1 AC does not fade easily, which helps during maintenance.
What stands out about this relay is the Warrick Controls name on the front.
The black color gives it a neutral presence that doesn't fight with your wire colors or labels.