I wondered about it being blood but it's a bit of a strange place for blood as it's in the air cell from which the veins would have receded already. So I'm not too sure what's going on there. Hopefully it's just one of those weird anomalies.
The bill will be quite apparent when the duckling...
As long as the duckling is still moving it sounds like it's fine, but if you feel like it isn't as responsive and it isn't making much noise when you talk to it, I would make a safety hole. 48 hours is the longest you can leave them.
If you do decide to make a safety hole (and that egg shell...