I'm working on a LED project similar to the one in the picture, and I need to identify the black wire that the arrows are pointing to. What are those called?
Here is a picture:
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Those are signal grounds, which in this case are connected to supply ground. They give the WS2812s the proper reference for the data signals on the colored lines in the twisted pairs.
It's not a wire, actually. It's an insulator. ;)
It could be heat-shrink tubing, which is slipped over the solder joint. Otherwise, the solder joint would be exposed and short against something. The tubes in the picture look like they weren't heated and shrunk.
It could also be black electric tape wound over the solder joint.
In case you mean the physical item rather than the logical one, it's a piece of insulating sleeving over a (probably) twisted and soldered join.