As I mentioned upthread: none of those are disallowed on common server platforms, but ironically canonical Base64 _will actually_ break, and it'll break rarely

As I mentioned upthread: none of those are disallowed on common server platforms, but ironically canonical Base64 _will actually_ break, and it'll break rarely enough that you're likely to miss it in testing! (The last character in canonical Base64, for symbol 63b10, is /, which is disallowed on (*nix) filesystems and does not occur in the native BlurHash dictionary. Unless you're suggesting encoding the encoded format, I guess.)