I have done this numerous times using Blockchain.com to transfer coins going back to 2014
That site is filled with bugs. I've seen a lot of topics about it lately. This may have happened to your uncompressed private key too:
Blockchain.info: In some cases, Blockchain.info (now .com) shows a compressed private key while the address was uncompressed. Read
this topic.
but now Electrum won't accept the password
That means you're using the wrong password.
and when I was opening the wallet, it did not ask me for a seed phrase
You said you're dealing with a private key from cold storage.
so I am sunk and utterly stunned! It's all lost.
Nothing is lost if you have your private key (and if you didn't leak it). Create a new Electrum wallet, import the key. Do this safely offline and go from there.