White vs black screen borders is mainly just a marketing choice, depending on which qualities of the product they want to emphasise. White is associated with cleanliness and elegance - it's the same reason most bathroom and kitchen appliances are white. Whereas black reminds us of the night sky snd hidden places, so we associate it with power and a sense of mystery.
There is an element of gendered marketing to it as well, with women being more likely to prefer white products and lighter colours, while men are more likely to prefer black and darker colours. Think of how men's and women's cosmetics products are marketed, and even things like clothes, food and books.
Does that mean new iMac is marketed for women and the old iMac was marketed for men? I think this could be partially true, but it's not the full picture. A bigger part of why Apples goes back and forth between the different colour scheblack and white is to stand out from the rest of the market, including their own products. When every iMac for a decade and a half has had black around the screen, the white one looks new and interesting.
We've seen this for example with the Macbook Air. It was silver for a decade, but when that became played out and cheap-looking, they made it black to look more like a Macbook Pro. When every other company was selling black cables and earphones, Apple made iPod cables and earphones white, which they emphasised in the marketing material. And when the first four generations of iPhone only came in black, they managed to create hype around a white iPhone 4 (coninciding nicely with a need to broaden the product's appeal to women).
You also saw this to some extent with the 2018 Mac Mini in Space Grey, but they probably also did this to match the aesthetic of the 'Pro' products. These are marketed to emphasise their power and ecause the professionals who buy them also tend to be mostly male, but even the women in professional settings would prefer something that looks at least as powerful as what their male coworkers are using. So black and dark colours are the general rule for the 'Pro' market.
Likewise, a replacement for the 27-inch will likely have at least one option for black around the screen. If they market it as an iMac Pro, then nearly all the colour options will include at least some black. I'd be willing to put money on it.