That's probably the beast, Kathleen. As you noted, there's apparently considerable variation in the coloration of the larvae. The book I have (a Princeton guide also by David L. Wagner) shows a pale green larva with setae almost exactly like the caterpillar we had in hand. Wagner states that prepupal forms of daggers often turn pink or red, "especially those that tunnel into wood to to fashion a pupal chamber." There are also melanistic forms, which might account for the black head (which also could have been a dark reddish brown.) That the larva might have been looking for a soft chunk of wood would make sense, because my grandson found it on the ground, and when I let it go it preferred being in the understory.
Thanks much !