I'm so sorry this has taken so long to get back to you, but I found a colleague who was having a similar issue with his own blog/phone and we were using that as a test to try and get things resolved!
We found that even though he had entered it correctly, the WordPress app may have been reading his WordPress.com blogs as having been entered as self-hosted WordPress.org blogs, which is what results in the 403 errors. The fix was the remove the blogs from the app, make sure that the app is updated, and then to re-add them, ensuring that we are adding them as WordPress.com-hosted blogs. That has allowed the blogs to work correctly!
Hopefully this will work for you as well!