Just thought I'd add what worked for me since I worked on this for over an hour and don't see my particular solution listed anywhere in this thread.
I have successfully built a certain maven project on my PC for years now, and have been training a colleague on how to build it himself on his mac. He kept getting blocked mirror errors, while I wasn't -- even though we are using the same version of maven (3.8.1) and the same exact project repository/branch.
We tried many of the suggestions in this thread (and elsewhere) but nothing was helping. I decided to see where our settings files differed. This was when I realized that he was using a local settings.xml file (located in his .m2 directory), and that I did not have that settings.xml file inside my .m2. I found that I was using the global settings.xml that had come with maven, in the conf directory under the main maven install directory (.../apache-maven-3.8.1/conf/settings.xml). I had never modified this config file. The global settings file included a mirrors section, but the local one did not.
I'm not really sure what sequence of events caused my colleague to have this user-specific settings file when I didn't -- perhaps something about the mac install vs PC.
So I had my colleague rename the .m2 settings file to settings.txt, in order to disable it and force maven to use the global one. The first try didn't work, but then when he edited the global settings.xml mirror section to have <blocked>false</blocked> instead of true: VOILA problem fixed. It built successfully on the next attempt.