Sorry to hear one passed.
Do you have pictures of the stool?
What are you feeding them?
Blood in the droppings can indicate several things; with ducklings, I would be most concerned with coccidiosis, caused by a parasite known as coccidia which attacks the inner intestinal lining, resulting in internal bleeding. There are also several bacteria we may encounter with poultry that can cause blood in the stools - clostridium perfringens is one to name as well.
In an ideal situation, you would collect some of the bloody fecal matter and have it brought to your vet for a fecal analysis; that would include gram staining - most birds will have gram-positive bacilli rods in their stools, if there seeing anything other than that, and in a large amount that's a good suggestion there is a bacterial problem at play. A fecal floatation may able be done to check for any coccidia currently in the stool.
Without such tests, treatment is founded on a conjecture basis, so you could possibly treat for the wrong thing, while the real problem advances. If you are not able to see a vet, I would do a broad treatment, and treat for both coccidiosis, and bacterial infection.
Here is information in regards to that:
https://www.backyardchickens.com/threads/updated-corid-and-amprol-amprolium-dosing.818879/
https://www.backyardchickens.com/threads/medications-for-small-chicks-and-ducklings.1441331/
https://allbirdproducts.com/products/baytril-10
Unfortunately, unless the problem is treated soon, ducklings often prove a poor prognosis as far as dealing with these problems due to their small size, and immunocompromised state of health.