I'm not saying this isn't an issue, but since it's only there with your login screen, is it a "significant" issue?
And of course I'll ask if you have cold rebooted since changing your background. The OS appears to cache a lot of stuff in places we normal humans don't have access to, so it may just be that that cache is still holding onto the blurred version of B. I'm particularly leaning this way because you've deleted the original file. So a cold reboot "should" change this. If you have cold rebooted, I don't know what the next step is.