Caterpillars come in a huge variety of species, and in many different colours, sizes and features. Some caterpillars will change into moths and others to butterflies and different species will have different diets. If you are trying to identify a large black species of caterpillar, you will still need to narrow it down from a list of large black caterpillars. Fortunately, with the right description and resources, it can be easy to identify the species of black caterpillar that you have found.
Note the time and location in which you found the caterpillars in a notebook. Include the kind of plant on which you located them, the geographic location and the month. If the caterpillars are eating anything, note what it is.
Describe the caterpillar in your notebook, including the size, any colours or patterns on its black surface, its fur or lack of fur, spines or any other distinguishing marks. Sketch a picture using a pencil if you can. If you have a camera, take a photo.
Contact the department of biology at a local college or university to find out if there are any entomologists working at the school who could help you identify the caterpillars. If no one is available with the necessary expertise, contact a local entomological society or a registered extermination company. If the species is a pest, exterminators will likely recognise it.
Check the pictures and descriptions of different kinds of caterpillars listed on the "Butterfly and Moth Caterpillars Identification Guide," the "Moth Photographers: Larvae of North American Lepidoptera" listing, or in the listings of different caterpillars found in the "Caterpillars of Eastern Forests" website. Check the descriptions and photos to find a caterpillar that matches the habitat, diet and description of the one you are analysing.
Contact the administrators of those websites. E-mail them a description of your caterpillar, including all the information you noted, along with any photos you may have.