The address with the bitcoin balance is a legacy bitcoin address that starts with 1. The address definitely has a positive BTC on it. The corresponding WIF private key is 52 characters and starts with L.
Since you never mentioned the imported address' format: if the imported address also starts with '1';
There's a chance here that Blockchain messed-up the WIF private key's encoding where it should be without the compressed byte flag.
a.k.a: uncompressed WIF private key, starts with '5'.
To encode it into that, you may use various offline tools but I'd recommend iancoleman's compression tool
Link (
do not use this online version, read "Offline Usage" below it):
https://iancoleman.io/bitcoin-key-compression/Then use the "
Private Key (WIF)" under "
Uncompressed".