that imports as
https://blockchain.info/address/1MtR9nBGfWBLY23q4dRGRjjzBBiEdxRmC3if they are not valid addresses then that is a fatal flaw in the blockchain because it's showing as a valid address and you could send coins there
If you edit the address it comes back as checksum not valid, so it clearly does check the addresses and says that this one is valid. If it's not then blockchain is fucked then
If you could import that invalid private key to begin with, blockchain.info is not verifying checksums properly. It's invalid, end of story. The address checksum is a different matter, you can have an address but no matching private key.