if it was a bad private key, you would get the error when trying to import it. and blockchain.info wouldnt be able to tell you the balance as it wont beable to read the private key to translate the public key.

so if its showing a balance but not letting you spend it.. then its nothing wrong with the private key. but instead how blockchain.info is coded.

i say this because your screenshot appears to be on the transaction page not the import private key page. so based on that i feel it is a fault on blockstreams part.

if however it is related to an error appearing when you import the key.. check you didnt copy it, but accidentally including a space before or after what you highlighted to copy, i have seen a few online services error out due to a simple space typed in before or after the key