Have you tried removing the passphrase and importing the unencrypted wallet into blockchain.info? I've never imported a wallet.dat there, so I don't know how it handles them.

The only other things I can think of that could be the problem is that either you copied the key wrong and it's missing a character (I've done that before), you're using the public address instead of the private key (unlikely, private key starts with a 5, bitaddress.org can give you an example and it's pretty obvious) or that all your coins are in a change address and blockchain.info won't sweep an empty key (bitaddress.org would have worked, though, so it's probably not that).

Have you tried telling bitcoin-qt to rescan the blockchain yet? If so, did it give any errors, and how long did it take?

Also, can you post the public address here? DON'T post the private key, just the address you use to receive funds for that wallet (e.g. 1M8oUcBnkRDEhWWgV8ZXLTB6p1mgnejVbX).