But, why would they have a private key in Base64? This is definitely not what's wrong here.
Bitaddress still gives private keys in Base64 format.
if it's a WIF it has to contain both uppercase and lowercase characters.
It can (and usually does), but it doesn't
have to. 5JAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABW8UEE6 is a perfectly valid WIF key, for example.