According to Newsblur UI, the Cloudflare Blog (https://blog.cloudflare.com/rss/) returns HTTP 503 only. If I try it with curl from my machine, I get a proper 200 with content that validates on validator.w3.org. Also, direct fetching from validator.w3.org works as well.
I have no idea what Newsblur could be doing differently to cause the 503. Maybe some blocking on Cloudflare side? @samuelclay, can you see some more helpful details in the response directly on your servers?
I worked with a couple folks at Cloudflare about a year ago to get NewsBlur placed on the allow list. That seems to have regressed, so I reached out to my contacts and will let you know when I hear back. Also, please let me know if it starts working again.
Just a short status update: at 2021-11-24 04:01:02, Newsblur once got a 301 redirect (can’t see the destination in the UI). Maybe somebody at Cloudflare is working on it?
Unfortunately, afterwards only 503 again 
Hopefully. I have a few feeds that are served on Cloudflare that are 503ing. I was going to wait a bit before complaining again.
As mentioned before, I can escalate within Cloudflare if you don’t get anywhere.