I use single quotes for items that, while technically a string, could be considered a value or symbol. For example:
syslog('debug',"Just opened %s for output",filename)
Some people make the case that it's easier to write single quotes (well, depending on the keyboard format anyway). For keyboards in the US standard you have to hold the Shift key to write a double quote. But the good thing about Black is that you can still write your code using single quote and when you run the command line utility it will fix/normalize the code to use double quotes.
Nowadays I got so used to it that I even write my Python code using double quotes. And looking at Python code using single quotes looks weird/unpleasant for me.