It's a color mutation most likely. Otherwise, the worm wouldn't be eating and moving.
http://gallery.pethobbyist.com/photo.php?id=113780
I've also seen a pic of a black hormworm being fed to a chameleon on a chameleon breeder page on Facebook once.... I'm skimming through a lot of breeder pages that I know off the top of my head trying to find it for you. It was on a breeder's Facebook page under a Halloween category in their albums...... if only I could find it....
What color is the horn? I can't see the horn on the black worm.... The above shows a mutation of a tobacco hornworm, as you can see both horns on both worms are red, an indication of a tobacco hornworm.
https://growingsmallfarms.ces.ncsu.edu/growingsmallfarms-hornworm/
That will show you a black tomato hornworm, with a blackish/blueish horn instead of red, an indication of a tomato hornworm. Tomato hornworms are typically green, just like the tobacco hornworms. Sometimes a mutation in either kind will cause them to turn black though.
https://entomologytoday.org/2013/12...ference-between-tomato-and-tobacco-hornworms/