After further research, I am pretty sure the infestation is the Hellebore Aphid (Macrosiphum hellebori). Please let me know if this is correct and give me advice on preventing a similar situation from occurring on my other, fortunately more distant, Hellebores. 

Several species of aphid are reported from hellebore. The most common is hellebore aphid, a whitish-green aphid that forms dense clusters on the leaves, stems, or flowers, sometimes forming dense colonies. Aphids are often visible, but sometimes are on the undersides of the leaves so plants should be checked from time to time. Damage includes abundant honeydew and sooty mold or blackening of leaves, or viruses transmitted when the aphids feed. The aphids are reported to overwinter as eggs, or as adults, and hatch in early spring. When the egg hatches, the aphid is called a stem mother. After maturing, she gives live birth to the young. They in turn mature and birth more live young. In this way, there may be waves of aphids of different sizes. Watch for signs of aphids: honeydew, sooty mold and the white shriveled caste off aphid exoskeletons, or for ants that are harvesting the honeydew. Syrphid fly larvae are found munching on the aphids in the spring. Often natural enemies and ants are noticed even before the aphids are seen. Aphids are partial to succulent new growth. They are common in spring, but are reported in fall and winter as well.

Hosing off aphids with plain water can work where plants are potted or isolated in the landscape.

Releases of lacewing larvae may have some success depending on the size of the aphid population.

Timing critical as many species have complex life cycles that alternate between asexual & sexual reproduction as well as summer and winter plant hosts. Adults, nymphs and winged adults found on succulent plant tissues. Monitor winged adults with yellow sticky traps. Scout landscape plants for honeydew and sooty mold. Dormant applications of horticultural oils are effective on managing the non-mobile life stages. Target the rapidly-increasing aphid populations early in the growing season, before leaves curl about and protect aphids from sprays. Most products are contact and thorough coverage is essential.

Home landscape products include: acetamiprid, azadirachtin, horticultural oils, imidacloprid, insecticidal soaps, kaolin clay, Malathion, pyrethrins, and spinosad.