Would be very pleased to see blockchain.info support smarter key importing. I have a giant set of lists of privkeys in base58 (separated by a space) I can't seem to do anything with unless I want to enter them in one by one in the import privkey section.
I'd swear I once imported a few keys by grabbing an unencrypted backup, editing my keys into the file, and restoring from the backup...took a look, try this out:
* log into your wallet
* select "Import/Export"
* select "Export Unencrypted"
* change key format to "Bitcoin-Qt format"
* copy the JSON blob into the editor of your choice
* add records for the private keys you want to add...the format should be self-explanatory
* select "Import Wallet"
* paste in your updated JSON blob and click the "Import Wallet" button down at the bottom
You probably want to make two copies of the backup and work on one of them, so that if you screw it up, you can restore from the original.
Also, you mentioned having a list of private keys...do you have the addresses associated with them? If not, you'll need to retrieve them.
Finally, blockchain.info warns that it'll only handle up to about 400 addresses for you. If your list is larger than that, this might not work for you.