The dark coloring of the housing might be a functional consideration. Computer software has been moving toward darker themes because it can be more relaxing on the eyes, in particular it lessens the appearance of vitreous floaters.
I think its more of a trend than anything.
Test equipment most likely stuck to bright beige colors because that is what computers back then looked like. And those all became that beige color because computers ware mostly meant for use in an office and standards ware being put down on what an office work environment should be like, among that was making this fancy new electronic equipment fit into the office environment. Offices are generally a bright and well lit environment so to fit in with the light colored desks and walls the computer equipment should have similar colors to fit in. Since offices bought lots of computers, the manufacturers made computers that fit into that, along with all the other peripherals like keyboards, mice, disk drives, printers etc... Then when people started buying computers for themselves they got the same beige colored boxes because that is what you could buy. And i suppose it just became the standard color for a computer to be beige, it looked weird if it was any other color. So i suppose test equipment drew from the same inspiration. Not all test equipment was light colored, but most of it was. So i suppose it also became weird for test equipment to be dark colored.
But then people started caring less about what color computers are in an office and manufacturers started experimenting with black computers for the sake of being different than everyone else. Especially once IBM started making black office workstations. They stood out and so everyone else also started making black computers to stand out until it became the new standard once everyone was making black computers. So apple wanting to stand out from all that went the other way in making completely white computers or even making brightly colorful computers to stand out even more.
Same thing happened when blue LEDs came around. They used to be expensive at first so only the fancy expensive equipment would have a blue power LED. But then everyone else also wanted to look fancy so once the price of blue LEDs came down everyone was sticking obnoxiously bright blue power LEDs on everything.
I think this fad is simply now catching on to test equipment where going black is an attempt to stand out and be hip. I personally don't like it so much, but hey everyone has there own taste. Possibly 15 years later all of it will be black and the beige ones will be considered old looking, like it is with computers.