Why are some UK caterpillars very dark or "Jet Black"?

[PhilBJohnson]

One thought was, that it was not so much to do with camouflage, but they gained a significant advantage in absorbing body warmth, when basking in direct sunlight. That might have meant being warm enough for a good meal, while air temperatures were still cool.

When butterflies warmed up with their wings open, there was a greater sunlit surface area that was warmed, compared with having their wings closed.