I now have an external blackbox working on my KISS FC v1. I want to share the steps I went through to get it working because the documentation is here and there.
I picked up an Openlogger from FliteTest, but I believe ReadyMadeRC and Amazon have models that will work as well. First I needed to flash a Cleanflight/Betaflight/KISS friendly version of the Openlogger firmware onto the logger. I did this with an FTDI usb to TTL adapter. Connecting all the corresponding pins between the adapter and the logger, and swapping RX and TX lines. Also important is that DTR goes to GRN, and CTS goes to BLK. A similar connection can be made if you use an Arduino Uno as the programmer. I downloaded the correct firmware from
here and followed the directions to move the given libraries to where Arduino can find them. I opened the Arduino sketch file and set the board to 'Arduino Uno', and the port to the port of my FTDI adapter. The programmer was left as the default. I pressed the upload button and it successfully flashed the logger. I then wired up the logger for use with the KISS FC using the diagrams found
here on the Flyduino site, and plugged it into port 2 of the FC. Finally, I plugged the FC into my computer, and in the advanced tab of the KISS configurator, I set the Serial Device to 'Logger 30%'. Initially I had it set to 100%, but I was getting gaps in the data when viewed in the Cleanflight Blackbox explorer (Betaflight Explorer didn't work). I run a loop time of 4khz so 30% is 1.2khz. A logging rate of 1khz is good enough, so that's what you should aim for when setting the logging percent.