Those people who selected the default colors for the Linux ls command must have much better eyes than I. E.g. I think that dark blue text on black background is very difficult to read, but apparently they think it is fine.
Fortunately it is easy to change these colors by setting the LS_COLORS environment variable. To make that change permanent, I put it into my .bashrc file like this:
LS_COLORS=$LS_COLORS:'di=1;35:' ; export LS_COLORS
I have put this here so I can easily find it whenever I need it. If anybody else finds this useful, you are welcome.