Hi Z!
What it's actually looking for by default is the presence of favicon.ico. Your website doesn't have that in the document root.
Just now I made it work the way that you were expecting. If favicon.ico isn't found it then checks for /b/img/blank.png.
blank.png is preferred.
Hope this helps.
Here's my result https://testmy.net/latency?tID=g5uvfbbcq3