Why is my note screen black, with text also black, but being projected onto a white blocky background?

How about just a plain white background with black text?

I can’t tell if this is a Catalina or DT thing.

DT 3

A screenshot would be useful, by default the text should black-on-white (light mode) or white-on-black (dark mode).

In that screenshot, OS X (Mojave) was in dark mode. When I switched it to Light, I have normal black text on a white background.

I presume that you pasted this text from a web page that has black text on white background? The Sorter uses the formatting that comes with the text. See also the font that it uses which is not the default font. You could use a tool like our WordService to clear the attributes.