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?