It's a known problem. Or at least, I know this transaction (from Nov 2) hasn't confirmed, I didn't know you get repeat notifications about it.

The API call to blockchain.info that I used so far used their default fee, which apparently isn't sufficient for this transaction. I've since started paying a higher fee. If the tx doesn't confirm I'll send another tx for the payment, and I hope the unconfirmed one will be forgotten eventually.