Minecraft Java puts a black drop-shadow behind text. This is normally fine for lighter colors, however for darker colors it can make the text more difficult to read.

For example, see the attached screenshot. Notice that the text is extremely hard to read, especially where there are small gaps in the text. This can be seen on the 's', which has the middle covered up like a '$' dollar sign.

/tellraw @s {"text": "The quick brown fox jumps over the lazy dog.", "color": "black"}

This applies to other text colours as well, but black demonstrates the issue especially well.

I find this issue is very bad on servers, enough so that I occasionally need to tell someone to use lighter colors because it's straight-up impossible for me to read text.