Based on what I’ve read, if it is Black Death or an infection, the vomit will smell really bad. So if you can, try to catch a whiff.
I don’t think it’s too uncommon for mantids to vomit occasionally without any infections or illnesses.
keep on the lookout for other symptoms, like lethargy and trouble walking/climbing, and definitely offer 100% raw honey and water. Also check your feeders. Crickets are the main carriers of these symptoms, so make sure that if you have any, they’re kept in healthy conditions.