I had a frustrating issue where prometheus blackbox exporter wasn’t able to check one of my websites and I couldn’t figure out why. I finally found this site which explained you can append &debug=true to the end of your probe in a URL string talking directly to blackbox. For example:

http://prometheus:9115/probe?module=http_2xx&target=http://customsite.com:8096/web/&debug=true

That finally got me to see what the problem was. It was resolving to an IPv6 address, but I didn’t have my IPv6 stack properly configured. I then discovered this site which led me to the solution: prefer IPv4. I appended this to my http_2xx config:

http_2xx:
  prober: http
  http:
    preferred_ip_protocol: ip4
...

That fixed the issue!