With the exception of some beneficial species of earthworms, seeing any type of worm in your garden usually leads to trouble in the form of damaged plants. Black worms adorned with orange heads are most likely a type of armyworm.
Two of the types of armyworms that come to mind are the fall armyworm and the true armyworm. Neither is solid black but both have predominately black, brown or gray bodies adorned with slight yellow or orange stripes and an amber to dark orange colored head.
Armyworms are not really worms at all, but caterpillars that eventually turn into moths. Adult armyworms average 1/2 inch to approximately one inch long. Fall armyworm generations occur every 23 to 25 days. The true armyworm completes a generation in 41 to 66 days.
Armyworms can be found in flower and vegetable gardens. In large number they damage crops from the Rocky Mountains eastward to the coastal Atlantic states. True armyworms cause garden damage during late spring and summer months, while fall armyworms damage late season crops.