Umbrian, you're seeing either:
- an early stage in the larval phase of a moderate-sized insect, at which time the caterpillars may not resemble the last (pre-pupation) stage - which is when they will look 'how they should' if you were to compare with photos in books or online;
or
- a later-stage of a small insect, which should be identifiable from photos but for the fact that the smaller insects are not as well represented in books and online.
So identification is in principle a little difficult, especially without photos. But it may happen that some folks will be able to make good suggestions based on personal knowledge.
Peacock butterflies have black larvae, but so do many other butterflies and moths, and many of them do not occur in plagues. Young caterpillars will almost always be on their food plant. The fact that yours are not, and that they are lethargic suggests they may be adult and looking for somewhere to pupate (like you'll often see large white butterflies' pupae on the sides of houses - where the larvae crawled having left their foodplants). The other reason for lethargy is parasitism; there are several wasps that lay their eggs inside insect larvae, which slowly die. They may also be away from their foodplants because they have eaten all the local ones and are looking for more to devour.
If you Google for "plague of black caterpillars", you'll see you are not alone. But ident. is still tricky.
This hit could be of use, since it makes suggetions for control -
http://www.amnet.net.au/~mpeaty/pests1.html.
Also, make sure it is not this type of caterpillar -
http://wattsupwiththat.com/2010/05/14/now-its-caterpillar-outbreaks-caused-by-global-warming/ 
Note that processionary moths do not 'process' until late in their development.
