not a stupid question as some of the terms used in the trade and also at fishing stores can be confusing and refer to different items.
as far as i know most bait stores don't carry live blackworms. live blackworms are generally purchased at an LFS or in bulk from a distributor. they are very small (maybe an inch or so) oligochaete worms...related to earthworms but much skinnier and smaller. they are a great source of nutrition for young fishes, old fishes, and stubborn (in terms of eating) fishes.
mixing up the african arow's diet with blackworms and other food items at this stage will keep them growing steadily as well as keep them very healthy and thick (they often arrive skinny when initially purchased from an LFS or supplier since they are usually around 2-3" long and generally not consistently eating while being held for shipping, etc).
hope that helps!--
--solomon