Monarchs, viceroys, mimicry, predation and misconceptions

I'm getting much better at identifying viceroy butterflies, as opposed to the very similar monarch. Viceroys are noticeably smaller, and they have a tell-tale thin black stripe towards the back of the hindwing.

You probably know that viceroys are regarded as mimics of monarchs, which are distateful to most potential predators due to their diet of milkweed plants as caterpillars. Incidentally, milkweed - and therefore the monarch that feeds on it - is not actually poisonous, as it is often portrayed, it is just very bitter and unpleasantly.
Except that ... I found out recently that it is not quite as simple as that (of course it isn't!) It was long thought that viceroys were an example of "Batesian mimicry", mimicry in which an edible animal is protected by its resemblance to a noxious one that is avoided by predators. However, it turns out that the viceroy caterpillars feed on willows, poplars and cottonwoods that are also very bitter and avoided by avian predators. This is therefore an example of Müllerian mimicry, a form of mimicry in which two or more noxious animals develop similar appearances as a shared protective device. (Müllerian mimicry is named after the German naturalist Fritz Müller; Batesian mimicry is named after the English naturalist Henry Walter Bates).
And talking of misconceptions, we learned while visiting the monarch sanctuaries in Mexico earlier this year that not all predators avoid monarchs: spiders, fire ants, some birds and wasps are all happy to indulge in a bitter meal from time to time at various points in the monarchs' peregrinations, while in the Mexican sanctuaries themselves they are still at risk of predation from black-eared mice, black-backed orioles and black-headed grosbeaks. I wonder what the "black" connection is?