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Black death pathogen is spread by insects, unrelated to too much water. There is no cure and plants should be dug up and destroyed. It shows up in hybrid hellebores and mostly in nurseries, not home gardens. Yours could be a fungal issue, black spot, which is related to rainy weather. Try removing the leaves as they become infected.

https://hgic.clemson.edu/lenten-roses/

https://www.gardeningknowhow.com/ornamental/flowers/hellebore/hellebore-black-death.htm

https://www.gardeningknowhow.com/ornamental/flowers/hellebore/hellebore-plant-problems.htm

Growing in a container requires some extra attention, such as more water in the summer. (All container plants in summer need more water than in-ground plants.) Hellebores need a rich soil, so if you use plain potting soil, add some compost to enrich the potting soil. It doesn't sound like a disease since you don't mention spots or other fungal symptoms. So, I would focus on the cultural requirements of hellebores.

Here are more tips:

https://www.gardeningknowhow.com/ornamental/flowers/hellebore/hellebore-plant-problems.htm

https://www.gardeningknowhow.com/ornamental/flowers/hellebore/growing-hellebore-in-containers.htm

https://www.gardeningknowhow.com/ornamental/flowers/hellebore/common-hellebore-diseases.htm

I would plant them now so they have time to establish before bloomtime in late winter/early spring.

https://www.gardeningknowhow.com/ornamental/flowers/hellebore/growing-hellebores.htm

Is there some type of conifer dropping needles onto the area? This could be the issue. When these break down into the soil, it becomes quite acidic. Many plants cannot handle this.

Since they don't normally experience many deficiencies, anything that appears as one will likely be a pH imbalance.

You may look to moving them, or trying to amend the soil. Taking the soil's pH will be a good start: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-ph-plants.htm

This article will help you to test for nutrient deficiencies: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm

This collection of articles will help you to start fixing your soil once you figure out what is off: https://www.gardeningknowhow.com/category/garden-how-to/soil-fertilizers

Your local extension service can help, if you are unable to test your soil: https://www.gardeningknowhow.com/extension-search

As long as you are comfortable with this method (which it seems like you are) then it will be just fine. The important thing is not to let the soil remain too saturated, for too long. It does need to dry out just a little bit on top.

I would recommend giving them just enough to soak up, and make sure that it does not sit in the saucer. That would indicate that the soil is oversaturated. As long as there is no water left standing after watering, you can do this with no problem.

Here is an article that will offer more information on the care of Hellebores: https://www.gardeningknowhow.com/ornamental/flowers/hellebore/growing-hellebores.htm

I would try treating it first! If you can't treat it, then you may need to destroy and solarize the soil.

First, you might try a fungicide. This article will help:

https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

If that doesn't take care of the issue, then you can try priming the soil with DOLOMITIC LIME and WETTABLE SULFUR. This will take care of most infections.

If all else fails, you will need to take out the installments, refresh the soil, and continue treating until something takes into the soil without contracting the disease.

If that fails to solve the problem, then selecting a completely unrelated species to plant in its place will usually work just fine. Many diseases are plant, or Genus specific.