Several species have black color. https://www.science.gov/topicpages/h/horsehair+worms+nematomorpha
Gordiids, or freshwater hairworms, are members of the phylum Nematomorpha that use terrestrial definitive hosts (arthropods) and live as adults in rivers, lakes, or streams. The genus Paragordius consists of 18 species
Nematomorpha are marine parasites.
I often advise Praziquantel and Albendazole, Vitamin D3, and Calcium. The Flatworm formula. I did read of one person who stopped and started. I got the impression this was the wrong thing to do. Often, it can take 3 worm life cycles to get all the eggs. Little is known of marine parasites, even less the treatment. I would say maybe 75% of infections of marine parasites are treated using Prazi and ALB.
One website suggests IVM and Prazi.
One website suggested lufenuron should be used prior to praziquantel.
The technique I use to identify a formula is to sample several low doses of meds, and identify which meds effect my species. Then combine the meds, take at nominal strength, and take for 3 life cycles before stopping. Then wait to see if they return, how long does it take for them return.
I also would test castor oil caps (2) and peppermint oil caps (6) for any effect. Some worms that are prehistoric do not react to meds, because their DNA does not respond to antiparasitic medications. Only organic oils work on these prehistoric parasites. Topical thyme oil or peppermint supreme oil should clear the location, if you can catch them. Start at the ankle and apply the oil slowly towards the toes.
Several people have reported that while treating for other parasites, large numbers of horse hair worms have exited the body.
People have also sent me videos of microscopic horse hair worms.
Remember there are thousands of species of parasites, so few have ideas on what it is. Only experience and persistence help in these situations.
Often a cocktail is required to cure an infection. One med for eggs, one med for junior stages, one med for adult. Many of my formulas are based on this three leg model.
For example, Hookworms can be treated using albendazole, mebendazole, and DEC.
Strongyles can be calmed by IVM, Fenbendazole slowly.
Black worms are usually a sign they are dying, except for horse hair worms that can be actually black in color.
Do not be surprised if a med actually causes them to exit the skin. That is usually when you are getting close in your formulation.