Blockchain.info has been known to have this bug for a long time.
It doesn't check the checksums and lengths of the WIF keys entered and that's quite a big problem.

The bug is hard to reproduce outside of BCI, so my suggestion would be creating a new wallet (remember the usual security precautions, and since that key has already seen BCI, it can be considered compromised), importing the troubled WIF key and exporting it back (I've tried that with an intentionally-corrupted key and it appears to work).